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A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical
data storage device File:Reel-to-reel recorder tc-630.jpg, On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data storage medium. Data storage is the recording (storing) of information (data) in a sto ...
that stores and retrieves
digital data Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware * Digital electronics, electronic circuits which operate using digital signals **Digital camera A digital camera is a camera that ...
using
magnetic storage Magnetic storage or magnetic recording is the storage of data Data are units of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its esse ...
and one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving
actuatorAn actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". An actuator requires a control signal and a source of energy In ...
arm, which read and write data to the platter surfaces. Data is accessed in a
random-access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any other ...
manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of
non-volatile storage Non-volatile memory (NVM) or non-volatile storage is a type of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of ...
, retaining stored data even when powered off. Modern HDDs are typically in the form of a small
rectangular box In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space tha ...
. Introduced by
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
in 1956, HDDs were the dominant
secondary storage A spindle of DVD-RW's. Computer data storage is a technology consisting of computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perfor ...
device for general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern era of servers and
personal computer File:Crystal Project computer.png, upright=0.9, An artist's depiction of a 2000s-era desktop-style personal computer, which includes a metal case with the computing components, a display monitor and a keyboard (mouse not shown) A personal com ...
s, though personal computing devices produced in large volume, like cell phones and tablets, rely on
flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses acti ...
storage devices. More than 224 companies have produced HDDs historically, though after extensive industry consolidation most units are manufactured by Seagate,
Toshiba is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, a sovereign sta ...
, and
Western Digital Western Digital Corporation (abbreviated WDC, commonly known as simply Western Digital and WD) is an American computer hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage ...
. HDDs dominate the volume of storage produced (
exabyte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
s per year) for servers. Though production is growing slowly (by exabytes shipped), sales revenues and unit shipments are declining because
solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data Persistence (computer science), persistently, typically using flash memory, and functioning as secondary storage in the Computer ...
s (SSDs) have higher data-transfer rates, higher areal storage density, somewhat better reliability, and much lower latency and access times.Hutchinson, Lee. (June 25, 2012
How SSDs conquered mobile devices and modern OSes
. Ars Technica. Retrieved January 7, 2013.
The revenues for SSDs, most of which use
NAND flash memory Flash memory is an Integrated circuit, electronic Non-volatile memory, non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the ...
, slightly exceed those for HDDs. Flash storage products had more than twice the revenue of hard disk drives . Though SSDs have four to nine times higher cost per bit, they are replacing HDDs in applications where speed, power consumption, small size, high capacity and durability are important. Cost per bit for SSDs is falling, and the price premium over HDDs has narrowed. The primary characteristics of an HDD are its capacity and
performance A performance is an act of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function. Management science In the work place, ...
. Capacity is specified in
unit prefixA unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. Units of various order of magnitude, sizes are commonly formed by the use of such prefixes. The Metric prefix, prefix ...
es corresponding to powers of : a 1-
terabyte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
(TB) drive has a capacity of
gigabyte The gigabyte () is a multiple of the unit byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for ...

gigabyte
s (GB; where 1 gigabyte = (109)
byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
s). Typically, some of an HDD's capacity is unavailable to the user because it is used by the
file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
and the computer
operating system An operating system (OS) is system software that manages computer hardware, computer software, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), sch ...

operating system
, and possibly inbuilt redundancy for error correction and recovery. Also there is confusion regarding storage capacity, since capacities are stated in decimal gigabytes (powers of 1000) by HDD manufacturers, whereas the most commonly used operating systems report capacities in powers of 1024, which results in a smaller number than advertised. Performance is specified by the time required to move the heads to a track or cylinder (average access time) adding the time it takes for the desired sector to move under the head (average
latency Latency or latent may refer to: Science and technology * Latent heat, energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process * Latent variable, a variable that is not directly observed but inferred i ...
, which is a function of the physical
rotational speed Rotational speed (also known as speed of revolution or rate of rotation), of an object rotating around an axis is the number of turns of the object divided by time, specified as revolutions per minute Revolutions per minute (abbreviated rpm ...
in
revolutions per minute Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is the number of Turn (geometry), turns in one minute. It is a unit of rotational speed or the frequency of rotation around a fixed axis. International ...
), and finally the speed at which the data is transmitted (data rate). The two most common form factors for modern HDDs are 3.5-
inch Measuring tape with inches The inch (symbol: in or ″) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete pie ...
, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard
interface Interface or interfacing may refer to: Academic journals * Interface (journal), ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Lin ...
cables such as PATA (Parallel ATA),
SATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a computer bus In computer architecture, a bus (a contraction of the Latin '' omnibus'', and historically also called ''data highway'') is a communication system that transfers d ...

SATA
(Serial ATA),
USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables and connectors and protocols for connection, communication and power supply (Interface (computing), interfacing) between computers, peripherals and ...

USB
or SAS (
Serial Attached SCSI In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...
) cables.


History

The first production IBM hard disk drive, the 350 disk storage, shipped in 1957 as a component of the IBM 305 RAMAC system. It was approximately the size of two medium-sized refrigerators and stored five million six-bit characters (3.75
megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units International is an adjective (also used as a noun) ...
s) on a stack of 52 disks (100 surfaces used). The 350 had a single arm with two read/write heads, one facing up and the other down, that moved both horizontally between a pair of adjacent platters and vertically from one pair of platters to a second set. Variants of the IBM 350 were the
IBM 355IBM manufactured magnetic disk storage devices from 1956 to 2003, when it sold its hard disk drive business to Hitachi. Both the hard disk drive (HDD) and floppy disk drive (FDD) were invented by IBM and as such IBM's employees were responsible for m ...
,
IBM 7300IBM manufactured magnetic disk storage devices from 1956 to 2003, when it sold its hard disk drive business to Hitachi. Both the hard disk drive (HDD) and floppy disk drive (FDD) were invented by IBM and as such IBM's employees were responsible for m ...
and
IBM 1405IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the ...
. In 1961 IBM announced, and in 1962 shipped, the IBM 1301 disk storage unit, which superseded the IBM 350 and similar drives. The 1301 consisted of one (for Model 1) or two (for model 2) modules, each containing 25 platters, each platter about thick and in diameter. While the earlier IBM disk drives used only two read/write heads per arm, the 1301 used an array of 48 heads (comb), each array moving horizontally as a single unit, one head per surface used. Cylinder-mode read/write operations were supported, and the heads flew about 250 micro-inches (about 6 µm) above the platter surface. Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning. The 1301 cabinet was about the size of three home refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes per module. Access time was about a quarter of a second. Also in 1962, IBM introduced the disk drive, which was about the size of a washing machine and stored two million characters on a removable
disk pack Disk or disc may refer to: * Disk (mathematics) In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathema ...
. Users could buy additional packs and interchange them as needed, much like reels of
magnetic tape Magnetic tape is a medium for magnetic recording, made of a thin, magnetizable coating on a long, narrow strip of plastic film Plastic film is a thin continuous polymeric material. Thicker plastic material is often called a "sheet". Thes ...

magnetic tape
. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called "fixed disk" drives. In 1963 IBM introduced the 1302, with twice the track capacity and twice as many tracks per cylinder as the 1301. The 1302 had one (for Model 1) or two (for Model 2) modules, each containing a separate comb for the first 250 tracks and the last 250 tracks. Some high-performance HDDs were manufactured with one head per track, ''e.g.'', Burroughs B-475 in 1964, IBM 2305 in 1970, so that no time was lost physically moving the heads to a track and the only latency was the time for the desired block of data to rotate into position under the head. Known as fixed-head or head-per-track disk drives, they were very expensive and are no longer in production. In 1973, IBM introduced a new type of HDD code-named "
Winchester Winchester is a cathedral city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. London: R ...
". Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism, but precluded removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of "Winchester technology" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. Later "Winchester" drives abandoned the removable media concept and returned to non-removable platters. In 1974 IBM introduced the swinging arm actuator, made feasible because the Winchester recording heads function well when skewed to the recorded tracks. The simple design of the IBM GV (Gulliver) drive, invented at IBM's UK Hursley Labs, became IBM's most licensed electro-mechanical invention of all time, the actuator and filtration system being adopted in the 1980s eventually for all HDDs, and still universal nearly 40 years and 10 Billion arms later. Like the first removable pack drive, the first "Winchester" drives used platters in diameter. In 1978 IBM introduced a swing arm drive, the IBM 0680 (Piccolo), with eight inch platters, exploring the possibility that smaller platters might offer advantages. Other eight inch drives followed, then drives, sized to replace the contemporary
floppy disk drive A floppy disk or floppy diskette (sometimes casually referred to as a floppy or diskette) is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a ...
s. The latter were primarily intended for the then fledgling personal computer (PC) market. Over time, as recording densities were greatly increased, further reductions in disk diameter to 3.5" and 2.5" were found to be optimum. Powerful rare earth magnet materials became affordable during this period, and were complementary to the swing arm actuator design to make possible the compact form factors of modern HDDs. As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s their cost had been reduced to the point where they were standard on all but the cheapest computers. Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as
Corvus Systems Corvus Systems was a technology company founded by Michael D'Addio and Mark Hahn in 1979 and located in San Jose, California, San Jose, Silicon Valley, in the United States. Corvus was a pioneer of the early days of personal computers, producing the ...
and Tallgrass Technologies, or under the PC system manufacturer's name such as the
Apple ProFile The ProFile (codenamed Pippin) was the first hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device File:Reel-to-reel recorder tc-630.jpg, On a reel-to-reel tape rec ...
. The
IBM PC/XT The IBM Personal Computer XT (model 5160, often shortened to PC XT) is the second computer in the IBM Personal Computer line, released on March 8, 1983. Except for the addition of a built-in Hard disk drive, hard drive and extra expansion slots, i ...
in 1983 included an internal 10 MB HDD, and soon thereafter internal HDDs proliferated on personal computers. External HDDs remained popular for much longer on the
Apple Macintosh The Macintosh (mainly Mac since 1998) is a family of personal computers designed, manufactured, and sold by Apple Inc. since January 1984. The original Macintosh is the first successful mass-market personal computer to have featured a graphic ...

Apple Macintosh
. Many Macintosh computers made between 1986 and 1998 featured a
SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and Peripheral, peripheral devices. The SCSI standards define SCSI command, commands, protocols, electrical, optica ...
port on the back, making external expansion simple. Older compact Macintosh computers did not have user-accessible hard drive bays (indeed, the
Macintosh 128K The Macintosh 128K, originally released as the Apple Macintosh, is the original Apple Inc., Apple Macintosh personal computer. Its beige case consisted of a CRT monitor and came with a keyboard and mouse. A handle built into the top of the case m ...
,
Macintosh 512K Mac 512K back panel The Macintosh 512K is a personal computer that was designed, manufactured and sold by Apple Inc., Apple Computer, inc. from September 1984 to April 1986. It is the first update to the original Macintosh 128K. It was virtual ...

Macintosh 512K
, and
Macintosh Plus The Macintosh Plus computer is the third model in the Macintosh line, introduced on January 16, 1986, two years after the Macintosh 128K, original Macintosh and a little more than a year after the Macintosh 512K, with a price tag of US$2599. As an ...

Macintosh Plus
did not feature a hard drive bay at all), so on those models external SCSI disks were the only reasonable option for expanding upon any internal storage. HDD improvements have been driven by increasing
areal density The area density (also known as areal density, surface density, superficial density, areic density, mass thickness, column density, or density thickness) of a two-dimensional object is calculated as the mass Mass is both a property Proper ...
, listed in the table above. Applications expanded through the 2000s, from the
mainframe computer A pair of IBM mainframes. On the left is the IBM z Systems z13. On the right is the IBM LinuxONE Rockhopper.">IBM_LinuxONE.html" ;"title="IBM z Systems z13. On the right is the IBM LinuxONE">IBM z Systems z13. On the right is the IBM LinuxONE R ...
s of the late 1950s to most
mass storage In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
applications including computers and consumer applications such as storage of entertainment content. In the 2000s and 2010s, NAND began supplanting HDDs in applications requiring portability or high performance. NAND performance is improving faster than HDDs, and applications for HDDs are eroding. In 2018, the largest hard drive had a capacity of 15 TB, while the largest capacity SSD had a capacity of 100 TB. , HDDs were forecast to reach 100 TB capacities around 2025, but the expected pace of improvement was pared back to 50 TB by 2026. Smaller form factors, 1.8-inches and below, were discontinued around 2010. The cost of solid-state storage (NAND), represented by
Moore's law Moore's law is the observation that Transistor count, the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and Forecasting, projection of a historical trend. Rather than a phy ...
, is improving faster than HDDs. NAND has a higher
price elasticity of demand A good's price elasticity of demand is a measure of how sensitive the quantity demanded of it is to its price. When the price rises, quantity demanded falls for almost any good, but it falls more for some than for others. The price elasticity giv ...

price elasticity of demand
than HDDs, and this drives market growth. During the late 2000s and 2010s, the
product life cycle In industry, product lifecycle management (PLM) is the process of managing the entire lifecycle of a product from inception, through engineering design and manufacture, to service and disposal of manufactured products. PLM integrates people, da ...

product life cycle
of HDDs entered a mature phase, and slowing sales may indicate the onset of the declining phase. The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013. In 2019,
Western Digital Western Digital Corporation (abbreviated WDC, commonly known as simply Western Digital and WD) is an American computer hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage ...
closed its last Malaysian HDD factory due to decreasing demand, to focus on SSD production. All three remaining HDD manufacturers have had decreasing demand for their HDDs since 2014.


Technology


Magnetic recording

A modern HDD records data by magnetizing a thin film of
ferromagnetic material Ferromagnetism is the basic mechanism by which certain materials (such as iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first tra ...
on both sides of a disk. Sequential changes in the direction of magnetization represent binary data
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both ...
s. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as
run-length limited Run-length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. RLL codes are defined by four main parameters: ''m'', ''n'', ''d'', ''k''. The first two, ''m'' ...
encoding, which determines how the data is represented by the magnetic transitions. A typical HDD design consists of a ' that holds flat circular disks, called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminum alloy, glass, or ceramic. They are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection. For reference, a standard piece of copy paper is thick. The platters in contemporary HDDs are spun at speeds varying from 4,200  RPM in energy-efficient portable devices, to 15,000 rpm for high-performance servers. The first HDDs spun at 1,200 rpm and, for many years, 3,600 rpm was the norm. , the platters in most consumer-grade HDDs spin at 5,400 or 7,200 RPM. Information is written to and read from a platter as it rotates past devices called read-and-write heads that are positioned to operate very close to the magnetic surface, with their flying height often in the range of tens of nanometers. The read-and-write head is used to detect and modify the magnetization of the material passing immediately under it. In modern drives, there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a
voice coil A voice coil (consisting of a bobbin, former, Collar (clothing), collar, and winding) is the coil of wire attached to the apex of a speaker driver, loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to ...
actuator or in some older designs a
stepper motor A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor#REDIRECT Brushless DC electric motor {{Redirect category shell, 1= {{R from other capitalisation ... that divides a full rotation into a number of equ ...

stepper motor
. Early hard disk drives wrote data at some constant bits per second, resulting in all tracks having the same amount of data per track but modern drives (since the 1990s) use
zone bit recordingIn computer storage File:Maxell DVD-RW 4.7GB crop 20051120.jpg, A spindle of DVD-RW's. Computer data storage is a technology consisting of computer components and Data storage device, recording media that are used to retain digital data (compu ...
– increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones. In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects⁠ ⁠— thermally induced magnetic instability which is commonly known as the "
superparamagnetic limit Superparamagnetism is a form of magnetism which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time betwee ...
". To counter this, the platters are coated with two parallel magnetic layers, separated by a three-atom layer of the non-magnetic element
ruthenium Ruthenium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

ruthenium
, and the two layers are magnetized in opposite orientation, thus reinforcing each other. Another technology used to overcome thermal effects to allow greater recording densities is
perpendicular recording Perpendicular recording (or perpendicular magnetic recording, PMR), also known as conventional magnetic recording (CMR), is a technology for data recording on magnetic media, particularly hard disks. It was first proven advantageous in 1976 by S ...
, first shipped in 2005, and used in certain HDDs. In 2004, a higher-density recording media was introduced, consisting of coupled soft and hard magnetic layers. So-called '' exchange spring media'' magnetic storage technology, also known as ''exchange coupled composite media'', allows good writability due to the write-assist nature of the soft layer. However, the thermal stability is determined only by the hardest layer and not influenced by the soft layer.


Components

A typical HDD has two electric motors: a spindle motor that spins the disks and an actuator (motor) that positions the read/write head assembly across the spinning disks. The disk motor has an external rotor attached to the disks; the stator windings are fixed in place. Opposite the actuator at the end of the head support arm is the read-write head; thin printed-circuit cables connect the read-write heads to
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power (physics), power of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It is a two-port networ ...

amplifier
electronics mounted at the pivot of the actuator. The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550 . The ' is a
permanent magnet Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated below with the iron filings A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsi ...
and
moving coil 250px, An Audio Technica AT-F3 moving coil phono cartridge A magnetic cartridge, more commonly called a phonograph cartridge or phono cartridge or (colloquially) a pickup, is an electromechanical transducer A transducer is a device that Energy ...
motor that swings the heads to the desired position. A metal plate supports a squat neodymium-iron-boron (NIB) high-flux
magnet Magnetic field#Magnetic field lines, Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated below with the iron filings A magnet is a material or object that produces a magnetic field. This magnetic ...

magnet
. Beneath this plate is the moving coil, often referred to as the ''
voice coil A voice coil (consisting of a bobbin, former, Collar (clothing), collar, and winding) is the coil of wire attached to the apex of a speaker driver, loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to ...
'' by analogy to the coil in
loudspeaker A loudspeaker (or ''speaker driver'', or most frequently just ''speaker'') is an electroacoustic transducer, that is, a device that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referre ...

loudspeaker
s, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives have only one magnet). The voice coil itself is shaped rather like an arrowhead and is made of doubly coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead (which point to the center of the actuator bearing) then interact with the
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

magnetic field
of the fixed magnet. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force. If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore, the surface of the magnet is half north pole and half south pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head. The HDD's electronics control the movement of the actuator and the rotation of the disk and perform reads and writes on demand from the
disk controller The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive. It also provides an interface between the disk drive and the bus connecting it to the rest of the sys ...
. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to feedback. These are either complete concentric circles (in the case of dedicated servo technology) or segments interspersed with real data (in the case of embedded servo technology). The servo feedback optimizes the signal-to-noise ratio of the GMR sensors by adjusting the voice coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media that have failed.


Error rates and handling

Modern drives make extensive use of
error correction code In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for error control, controlling errors in data over unreliable or noisy communication channels. The cen ...
s (ECCs), particularly
Reed–Solomon error correction Reed–Solomon codes are a group of error-correcting code In information theory Information theory is the scientific study of the quantification, storage, and communication Communication (from Latin ''communicare'', meaning "to share ...
. These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity. For example, a typical 1  TB hard disk with 512-byte sectors provides additional capacity of about 93  GB for the ECC data. In the newest drives, ,
low-density parity-check code In information theory, a low-density parity-check (LDPC) code is a linear error correcting code, a method of transmitting a message over a noisy transmission channel. An LDPC is constructed using a sparse Tanner graph (subclass of the bipart ...
s (LDPC) were supplanting Reed–Solomon; LDPC codes enable performance close to the
Shannon Limit In information theory Information theory is the scientific study of the quantification, storage, and communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject ( ...
and thus provide the highest storage density available. Typical hard disk drives attempt to "remap" the data in a physical sector that is failing to a spare physical sector provided by the drive's "spare sector pool" (also called "reserve pool"), while relying on the ECC to recover stored data while the number of errors in a bad sector is still low enough. The (Self-Monitoring, Analysis and Reporting Technology) feature counts the total number of errors in the entire HDD fixed by ECC (although not on all hard drives as the related S.M.A.R.T attributes "Hardware ECC Recovered" and "Soft ECC Correction" are not consistently supported), and the total number of performed sector remappings, as the occurrence of many such errors may predict an HDD failure. The "No-ID Format", developed by IBM in the mid-1990s, contains information about which sectors are bad and where remapped sectors have been located. Only a tiny fraction of the detected errors end up as not correctable. Examples of specified uncorrected bit read error rates include: *2013 specifications for enterprise SAS disk drives state the error rate to be one uncorrected bit read error in every 1016 bits read, *2018 specifications for consumer SATA hard drives state the error rate to be one uncorrected bit read error in every 1014 bits. Within a given manufacturers model the uncorrected bit error rate is typically the same regardless of capacity of the drive. The worst type of errors are
silent data corruption Photo data corruption; in this case, a result of a failed data recovery from a hard disk drive Data corruption refers to errors in computer data that occur during writing, reading, storage, transmission, or processing, which introduce unintended ...
s which are errors undetected by the disk firmware or the host operating system; some of these errors may be caused by hard disk drive malfunctions while others originate elsewhere in the connection between the drive and the host.


Development

The rate of areal density advancement was similar to
Moore's law Moore's law is the observation that Transistor count, the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and Forecasting, projection of a historical trend. Rather than a phy ...
(doubling every two years) through 2010: 60% per year during 1988–1996, 100% during 1996–2003 and 30% during 2003–2010. Speaking in 1997,
Gordon Moore Gordon Earle Moore (born January 3, 1929) is an American businessman, engineer, and the co-founder and chairman emeritus of Intel Corporation. He is also the author of Moore's law Moore's law is the observation that Transistor count, ...

Gordon Moore
called the increase "flabbergasting", while observing later that growth cannot continue forever. Price improvement decelerated to −12% per year during 2010–2017, as the growth of areal density slowed. The rate of advancement for areal density slowed to 10% per year during 2010–2016, and there was difficulty in migrating from perpendicular recording to newer technologies. As bit cell size decreases, more data can be put onto a single drive platter. In 2013, a production desktop 3 TB HDD (with four platters) would have had an areal density of about 500 Gbit/in2 which would have amounted to a bit cell comprising about 18 magnetic grains (11 by 1.6 grains). Since the mid-2000s areal density progress has been challenged by a
superparamagnetic Superparamagnetism is a form of magnetism Magnetism is a class of physical phenomena that are mediated by magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, ...
trilemma involving grain size, grain magnetic strength and ability of the head to write. In order to maintain acceptable signal to noise smaller grains are required; smaller grains may self-reverse (
electrothermal instability __NOTOC__ The electrothermal instability (also known as ionization instability, non-equilibrium instability or Velikhov instability in the literature) is a magnetohydrodynamics, magnetohydrodynamic (MHD) instability appearing in Plasma (physics)#Ma ...
) unless their magnetic strength is increased, but known write head materials are unable to generate a strong enough magnetic field sufficient to write the medium in the increasingly smaller space taken by grains. Magnetic storage technologies are being developed to address this trilemma, and compete with
flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses acti ...
–based
solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data Persistence (computer science), persistently, typically using flash memory, and functioning as secondary storage in the Computer ...
s (SSDs). In 2013, Seagate introduced
shingled magnetic recording Shingled magnetic recording (SMR) is a magnetic storage data recording technology used in hard disk drives (HDDs) to increase storage density and overall per-drive storage capacity. Conventional hard disk drives record data by writing non-overlap ...
(SMR), intended as something of a "stopgap" technology between PMR and Seagate's intended successor
heat-assisted magnetic recording Heat-assisted magnetic recording (HAMR) is a magnetic storage technology for greatly increasing the amount of data that can be stored on a magnetic device such as a hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed ...
(HAMR), SMR utilises overlapping tracks for increased data density, at the cost of design complexity and lower data access speeds (particularly write speeds and
random access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any other ...
4k speeds). By contrast,
HGST HGST, Inc. (Hitachi Global Storage Technologies) was a manufacturer of hard disk drives, solid-state drives, and external storage In computing, external storage comprises devices that store information outside a computer. Such devices may be ...
(now part of
Western Digital Western Digital Corporation (abbreviated WDC, commonly known as simply Western Digital and WD) is an American computer hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage ...
) focused on developing ways to seal
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining") ...

helium
-filled drives instead of the usual filtered air. Since
turbulence In fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the study of air and other gases ...
and
friction Friction is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related en ...

friction
are reduced, higher areal densities can be achieved due to using a smaller track width, and the energy dissipated due to friction is lower as well, resulting in a lower power draw. Furthermore, more platters can be fit into the same enclosure space, although helium gas is notoriously difficult to prevent escaping. Thus, helium drives are completely sealed and do not have a breather port, unlike their air-filled counterparts. Other recording technologies are either under research or have been commercially implemented to increase areal density, including Seagate's
heat-assisted magnetic recording Heat-assisted magnetic recording (HAMR) is a magnetic storage technology for greatly increasing the amount of data that can be stored on a magnetic device such as a hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed ...
(HAMR). HAMR requires a different architecture with redesigned media and read/write heads, new lasers, and new near-field optical transducers. HAMR is expected to ship commercially in late 2020 or 2021. Technical issues delayed the introduction of HAMR by a decade, from earlier projections of 2009, 2015, 2016, and the first half of 2019. Some drives have adopted dual independent actuator arms to increase read/write speeds and compete with SSDs. HAMR's planned successor,
bit-patterned recording Patterned media (also known as bit-patterned media or BPM) is a potential future hard disk drive technology to record data in magnetic islands (one bit per island), as opposed to current hard disk drive technology where each bit is stored in 20-30 ...
(BPR), has been removed from the roadmaps of Western Digital and Seagate. Western Digital's microwave-assisted magnetic recording (MAMR), also referred to as energy-assisted magnetic recording (EAMR), was sampled in 2020, with the first EAMR drive, the Ultrastar HC550, shipping in late 2020. Two-dimensional magnetic recording (TDMR) and "current perpendicular to plane"
giant magnetoresistance Giant magnetoresistance (GMR) is a quantum mechanics, quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert ...
(CPP/GMR) heads have appeared in research papers. A 3D-actuated vacuum drive (3DHD) concept has been proposed. The rate of areal density growth has dropped below the historical Moore's law rate of 40% per year. Depending upon assumptions on feasibility and timing of these technologies, Seagate forecasts that areal density will grow 20% per year during 2020–2034.


Capacity

The highest-capacity HDDs shipping commercially in 2021 are 20 TB. The capacity of a hard disk drive, as reported by an operating system to the end user, is smaller than the amount stated by the manufacturer for several reasons: the operating system using some space, use of some space for data redundancy, and space use for file system structures. Also the difference in capacity reported in SI decimal prefixed units vs.
binary prefixes A binary prefix is a unit prefix for multiples of Units of measurement, units in data processing, data transmission, and digital information, notably the bit and the byte, to indicate multiplication by a Exponentiation, power of 2. The compute ...
can lead to a false impression of missing capacity.


Calculation

Modern hard disk drives appear to their host controller as a contiguous set of logical blocks, and the gross drive capacity is calculated by multiplying the number of blocks by the block size. This information is available from the manufacturer's product specification, and from the drive itself through use of operating system functions that invoke low-level drive commands. Some older drives, e.g.,
IBM 1301IBM manufactured magnetic disk storage devices from 1956 to 2003, when it sold its hard disk drive business to Hitachi. Both the hard disk drive (HDD) and floppy disk drive (FDD) were invented by IBM and as such IBM's employees were responsible for m ...
, CKD, have variable length records and the capacity calculation must take into account the characteristics of the records. Some newer DASD simulate CKD, and the same capacity formulae apply. The gross capacity of older sector-oriented HDDs is calculated as the product of the number of
cylinders A cylinder (from Greek language, Greek κύλινδρος – ''kulindros'', "roller", "tumbler") has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. It is the idealized version of a solid physi ...
per recording zone, the number of bytes per sector (most commonly 512), and the count of
zones Zone or The Zone may refer to: Places Climate and altitude zones * Death zone (originally the lethal zone), altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span * Frigid zone, a ...
of the drive. Some modern SATA drives also report
cylinder-head-sector Cylinder, head, and sector of a hard drive. Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a hard disk drive. It is a 3D-coordinate system made out of a vertical coordinate ''head'', a horizo ...
(CHS) capacities, but these are not physical parameters because the reported values are constrained by historic operating system interfaces. The C/H/S scheme has been replaced by
logical block addressing Logical block addressing (LBA) is a common scheme used for specifying the location of Block (data storage), blocks of data stored on computer storage devices, generally secondary storage systems such as hard disk drives. LBA is a particularly simpl ...
(LBA), a simple linear addressing scheme that locates blocks by an integer index, which starts at LBA 0 for the first block and increments thereafter. When using the C/H/S method to describe modern large drives, the number of heads is often set to 64, although a typical modern hard disk drive has between one and four platters. In modern HDDs, spare capacity for defect management is not included in the published capacity; however, in many early HDDs a certain number of sectors were reserved as spares, thereby reducing the capacity available to the operating system. Furthermore, many HDDs store their firmware in a reserved service zone, which is typically not accessible by the user, and is not included in the capacity calculation. For
RAID Raid or RAID may refer to: Attack * Raid (military) Raiding, also known as depredation, is a military tactics, military tactic or operational warfare mission which has a specific purpose. Raiders do not capture and hold a location, but quickly r ...
subsystems, data integrity and fault-tolerance requirements also reduce the realized capacity. For example, a RAID 1 array has about half the total capacity as a result of data mirroring, while a RAID 5 array with drives loses of capacity (which equals to the capacity of a single drive) due to storing parity information. RAID subsystems are multiple drives that appear to be one drive or more drives to the user, but provide fault tolerance. Most RAID vendors use
checksum A checksum is a small-sized block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Car ...

checksum
s to improve data integrity at the block level. Some vendors design systems using HDDs with sectors of 520 bytes to contain 512 bytes of user data and eight checksum bytes, or by using separate 512-byte sectors for the checksum data. Some systems may use hidden partitions for system recovery, reducing the capacity available to the end user without knowledge of special disk partitioning utilities like
diskpart In computing, diskpart is a command-line interface, command-line disk partitioning utility included in Windows 2000 and later Microsoft operating systems, replacing its predecessor, fdisk. The command is also available in ReactOS. Overview Th ...

diskpart
in
Windows Microsoft Windows, commonly referred to as Windows, is a group of several proprietary {{Short pages monitor