A hard disk drive (HDD), hard disk, hard drive or fixed disk[b] is a
data storage device that uses magnetic storage to store and retrieve
digital information using one or more rigid rapidly rotating disks
(platters) coated with magnetic material. The platters are paired with
magnetic heads, usually arranged on a moving actuator arm, which read
and write data to the platter surfaces. Data is accessed in a
random-access manner, meaning that individual blocks of data can be
stored or retrieved in any order and not only sequentially. HDDs are a
type of non-volatile storage, retaining stored data even when powered
IBM in 1956, HDDs became the dominant secondary
storage device for general-purpose computers by the early 1960s.
Continuously improved, HDDs have maintained this position into the
modern era of servers and personal computers. More than 200 companies
have produced HDDs historically, though after extensive industry
consolidation most current units are manufactured by Seagate, Toshiba,
and Western Digital. HDD unit shipments and sales revenues are
declining, though production (exabytes per year) is growing. Flash
memory has a growing share of the market for secondary storage, in the
form of solid-state drives (SSDs). SSDs have higher data-transfer
rates, higher areal storage density, better reliability, and much
lower latency and access times. Though SSDs have higher
cost per bit, they are replacing HDDs where speed, power consumption,
small size, and durability are important.
The primary characteristics of an HDD are its capacity and
performance. Capacity is specified in unit prefixes corresponding to
powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000
gigabytes (GB; where 1 gigabyte = 1 billion bytes). Typically, some of
an HDD's capacity is unavailable to the user because it is used by the
file system and the computer operating system, and possibly inbuilt
redundancy for error correction and recovery. Performance is specified
by the time required to move the heads to a track or cylinder (average
access time) plus the time it takes for the desired sector to move
under the head (average latency, which is a function of the physical
rotational speed in revolutions per minute), 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, for
desktop computers, and 2.5-inch, primarily for laptops. HDDs are
connected to systems by standard interface cables such as PATA
SATA (Serial ATA),
USB or SAS (Serial Attached SCSI)
2.1 Magnetic recording
2.3 Error rates and handling
4 Price evolution
5 Form factors
6 Performance characteristics
6.2 Data transfer rate
6.3 Other considerations
7 Access and interfaces
8 Integrity and failure
9 Market segments
10 Manufacturers and sales
11 Competition from solid-state drives
12 External hard disk drives
13 See also
16 Further reading
17 External links
Video of modern HDD operation (cover removed)
Main article: History of hard disk drives
Improvement of HDD characteristics over time
Started with (1956)
Developed to (2017)
68 cubic feet (1.9 m3)[c]
2.1 cubic inches (34 cm3)[d]
2,000 pounds (910 kg)
2.2 ounces (62 g)
Average access time
approx. 600 milliseconds
2.5 ms to 10 ms; RW RAM dependent
US$9,200 per megabyte (1961)
US$0.032 per gigabyte by 2015
2,000 bits per square inch
1.3 terabits per square inch in 2015
~2000 hrs MTBF
~2500000 hrs MTBF
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 megabytes) on a stack of 50
In 1962 the IBM 350 was superseded by the IBM 1301 disk
storage unit, which consisted of 50 platters, each about 1/8-inch
thick and 24 inches in diameter. While the IBM 350 used only
two read/write heads , the 1301 used an array of heads, one per
platter, moving as a single unit. 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.
Access time was about a quarter
of a second.
Also in 1962,
IBM introduced the model 1311 disk drive, which was
about the size of a washing machine and stored two million characters
on a removable disk pack. Users could buy additional packs and
interchange them as needed, much like reels of 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"
Some high-performance HDDs were manufactured with one head per track
(e.g. IBM 2305 in 1970) so that no time was lost physically
moving the heads to a track. Known as fixed-head or head-per-track
disk drives they were very expensive and are no longer in
IBM introduced a new type of HDD code-named "Winchester". 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.
Like the first removable pack drive, the first "Winchester" drives
used platters 14 inches (360 mm) in diameter. A few years later,
designers were exploring the possibility that physically smaller
platters might offer advantages. Drives with non-removable eight-inch
platters appeared, and then drives that used a 5 1⁄4 in
(130 mm) form factor (a mounting width equivalent to that used by
contemporary floppy disk drives). The latter were primarily intended
for the then-fledgling personal computer (PC) market.
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
Corvus Systems and Tallgrass Technologies, or under the PC system
manufacturer's name such as the Apple ProFile. The
IBM PC/XT 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.
Many Macintosh computers made between 1986 and 1998 featured a SCSI
port on the back, making external expansion simple. Older compact
Macintosh computers did not have user-accessible hard drive bays
(indeed, the Macintosh 128K, Macintosh 512K, and
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
The 2011 Thailand floods damaged the manufacturing plants and
impacted hard disk drive cost adversely between 2011 and 2013.
Driven by ever increasing areal density since their invention, HDDs
have continuously improved their characteristics; a few highlights are
listed in the table above. At the same time, market application
expanded from mainframe computers of the late 1950s to most mass
storage applications including computers and consumer applications
such as storage of entertainment content.
Magnetic cross section & frequency modulation encoded binary data
See also: magnetic storage
A modern HDD records data by magnetizing a thin film of ferromagnetic
material[e] on a disk. Sequential changes in the direction of
magnetization represent binary data bits. 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
encoding,[f] which determines how the data is represented by the
A typical HDD design consists of a spindle that holds flat circular
disks, also called platters, which hold the recorded data. The
platters are made from a non-magnetic material, usually aluminum
alloy, glass, or ceramic, and 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 0.07–0.18 millimeters
Diagram labeling the major components of a computer HDD
Recording of single magnetisations of bits on a 200 MB
HDD-platter (recording made visible using CMOS-MagView).
Longitudinal recording (standard) & perpendicular recording
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. As of December 2013, the platters in most consumer-grade
HDDs spin at either 5,400 rpm 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
actuator or in some older designs a 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 recording – 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". To counter this, the platters
are coated with two parallel magnetic layers, separated by a 3-atom
layer of the non-magnetic element 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, first shipped in
2005, and as of 2007 the technology was used in many
In 2004, a new concept was introduced to allow further increase of the
data density in magnetic recording, using recording media consisting
of coupled soft and hard magnetic layers. That so-called exchange
spring media, 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.
HDD with disks and motor hub removed exposing copper colored stator
coils surrounding a bearing in the center of the spindle motor. Orange
stripe along the side of the arm is thin printed-circuit cable,
spindle bearing is in the center and the actuator is in the upper left
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 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 g.
Head stack with an actuator coil on the left and read/write heads on
Close-up of a single read-write head, showing the side facing the
The actuator is a permanent magnet and moving coil motor that swings
the heads to the desired position. A metal plate supports a squat
neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the
moving coil, often referred to as the voice coil by analogy to the
coil in loudspeakers, 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 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 actuator
bearing center) then interact with 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. Feedback of the drive electronics is accomplished by
means of special segments of the disk dedicated to servo 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
which have failed.
Error rates and handling
Modern drives make extensive use of error correction codes (ECCs),
particularly Reed–Solomon error correction. 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, as of 2009, low-density parity-check codes
(LDPC) were supplanting Reed–Solomon; LDPC codes enable performance
close to the
Shannon Limit and thus provide the highest storage
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 S.M.A.R.T (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 ends up as not
correctable. For example, specification for an enterprise SAS disk (a
model from 2013) estimates this fraction to be one uncorrected error
in every 1016 bits, and another SAS enterprise disk from 2013
specifies similar error rates. Another modern (as of 2013)
SATA disk specifies an error rate of less than 10
non-recoverable read errors in every 1016 bits.[needs update?] An
enterprise disk with a
Fibre Channel interface, which uses 520 byte
sectors to support the
Data Integrity Field standard to combat data
corruption, specifies similar error rates in 2005.
The worst type of errors are silent data corruptions 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.
Leading-edge hard disk drive areal densities from 1956 through 2009
compared to Moore's law
The rate of areal density advancement was similar to Moore's law
(doubling every two years) through 2010: 60% per year during
1988–1996, 100% during 1996–2003 and 30% during 2003–2010.
Gordon Moore (1997) 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
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 increasingly been challenged by a superparamagnetic
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) unless their magnetic strength is
increased, but known write head materials are unable to generate a
magnetic field sufficient to write the medium. Several new magnetic
storage technologies are being developed to overcome or at least abate
this trilemma and thereby maintain the competitiveness of HDDs with
respect to products such as flash memory-based solid-state drives
In 2013, Seagate introduced one such technology, shingled magnetic
recording (SMR). Additionally, SMR comes with design complexities
that may cause reduced write performance. Other new recording
technologies that, as of 2016[update], still remain under development
include heat-assisted magnetic recording (HAMR),
microwave-assisted magnetic recording (MAMR), two-dimensional
magnetic recording (TDMR), bit-patterned recording (BPR),
and "current perpendicular to plane" giant magnetoresistance (CPP/GMR)
The rate of areal density growth has dropped below the historical
Moore's law rate of 40% per year, and the deceleration is expected to
persist through at least 2020. Depending upon assumptions on
feasibility and timing of these technologies, the median forecast by
industry observers and analysts for 2020 and beyond for areal density
growth is 20% per year with a range of 10–30%. The
achievable limit for the HAMR technology in combination with BPR and
SMR may be 10 Tbit/in2, which would be 20 times higher than
the 500 Gbit/in2 represented by 2013 production desktop HDDs. As
of 2015, HAMR HDDs have been delayed several years, and are expected
in 2018. They require a different architecture, with redesigned media
and read/write heads, new lasers, and new near-field optical
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 can lead to a false impression of
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.
The gross capacity of older HDDs is calculated as the product of the
number of cylinders per recording zone, the number of bytes per sector
(most commonly 512), and the count of zones of the drive.[citation
needed] Some modern
SATA drives also report cylinder-head-sector (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 (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
hard disk drive, as of 2013[update], has between one and four
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.
RAID 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 x drives loses 1/x 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 checksums 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.
Main article: Disk formatting
Data is stored on a hard drive in a series of logical blocks. Each
block is delimited by markers identifying its start and end, error
detecting and correcting information, and space between blocks to
allow for minor timing variations. These blocks often contained 512
bytes of usable data, but other sizes have been used. As drive density
increased, an initiative known as
Advanced Format extended the block
size to 4096 bytes of usable data, with a resulting significant
reduction in the amount of disk space used for block headers, error
checking data, and spacing.
The process of initializing these logical blocks on the physical disk
platters is called low-level formatting, which is usually performed at
the factory and is not normally changed in the field. High-level
formatting writes data structures used by the operating system to
organize data files on the disk. This includes writing partition and
file system structures into selected logical blocks. For example, some
of the disk space will be used to hold a directory of disk file names
and a list of logical blocks associated with a particular file.
Examples of partition mapping scheme include
Master boot record
Master boot record (MBR)
GUID Partition Table
GUID Partition Table (GPT). Examples of data structures stored on
disk to retrieve files include the
File Allocation Table (FAT) in the
DOS file system and inodes in many
UNIX file systems, as well as other
operating system data structures (also known as metadata). As a
consequence, not all the space on an HDD is available for user files,
but this system overhead is usually small compared with user data.
See also: binary prefix § disk drives
Decimal and binary unit prefixes interpretation
Capacity advertised by manufacturers[g]
Capacity expected by some consumers[h]
macOS ver 10.6+[g]
1,000 GB, 1,000,000 MB
The total capacity of HDDs is given by manufacturers using SI decimal
prefixes such as gigabytes (1 GB = 1,000,000,000 bytes)
and terabytes (1 TB = 1,000,000,000,000 bytes).
This practice dates back to the early days of computing; by the
1970s, "million", "mega" and "M" were consistently used in the decimal
sense for drive capacity. However, capacities of memory
are quoted using a binary interpretation of the prefixes, i.e. using
powers of 1024 instead of 1000.
Software reports hard disk drive or memory capacity in different forms
using either decimal or binary prefixes. The Microsoft
of operating systems uses the binary convention when reporting storage
capacity, so an HDD offered by its manufacturer as a 1 TB drive
is reported by these operating systems as a 931 GB HDD. Mac OS X
10.6 ("Snow Leopard") uses decimal convention when reporting HDD
capacity. The default behavior of the df command-line utility on
Linux is to report the HDD capacity as a number of 1024-byte
The difference between the decimal and binary prefix interpretation
caused some consumer confusion and led to class action suits against
HDD manufacturers. The plaintiffs argued that the use of decimal
prefixes effectively misled consumers while the defendants denied any
wrongdoing or liability, asserting that their marketing and
advertising complied in all respects with the law and that no class
member sustained any damages or injuries.
HDD price per byte improved at the rate of −40% per year during
1988–1996, −51% per year during 1996–2003, and −34% per year
during 2003–2010. The price improvement decelerated to
−13% per year during 2011–2014, as areal density increase slowed
2011 Thailand floods
2011 Thailand floods damaged manufacturing facilities.
Main article: List of hard disk drive form factors
8-, 5.25-, 3.5-, 2.5-, 1.8- and 1-inch HDDs, together with a ruler to
show the length of platters and read-write heads
A newer 2.5-inch (63.5 mm) 6,495 MB HDD compared to an older
5.25-inch full-height 110 MB HDD
IBM's first hard drive, the IBM 350, used a stack of fifty
24-inch platters and was of a size comparable to two large
refrigerators. In 1962,
IBM introduced its model 1311 disk, which used
six 14-inch (nominal size) platters in a removable pack and was
roughly the size of a washing machine. This became a standard platter
size and drive form-factor for many years, used also by other
IBM 2314 used platters of the same size in an
eleven-high pack and introduced the "drive in a drawer" layout,
although the "drawer" was not the complete drive.
Later drives were designed to fit entirely into a chassis that would
mount in a 19-inch rack. Digital's
RK05 and RL01 were early examples
using single 14-inch platters in removable packs, the entire drive
fitting in a 10.5-inch-high rack space (six rack units). In the
mid-to-late 1980s the similarly sized
Fujitsu Eagle, which used
(coincidentally) 10.5-inch platters, was a popular product.
Such large platters were never used with microprocessor-based systems.
With increasing sales of microcomputers having built in floppy-disk
drives (FDDs), HDDs that would fit to the FDD mountings became
desirable. Thus HDD Form factors, initially followed those of 8-inch,
5.25-inch, and 3.5-inch floppy disk drives. Although referred to by
these nominal sizes, the actual sizes for those three drives
respectively are 9.5", 5.75" and 4" wide. Because there were no
smaller floppy disk drives, smaller HDD form factors developed from
product offerings or industry standards. 2.5-inch drives are actually
As of 2012[update], 2.5-inch and 3.5-inch hard disks were the most
popular sizes. By 2009, all manufacturers had discontinued the
development of new products for the 1.3-inch, 1-inch and 0.85-inch
form factors due to falling prices of flash memory, which
has no moving parts. While nominal sizes are in inches, actual
dimensions are specified in millimeters.
Main article: hard disk drive performance characteristics
The factors that limit the time to access the data on an HDD are
mostly related to the mechanical nature of the rotating disks and
Seek time is a measure of how long it takes the head assembly to
travel to the track of the disk that contains data. The first HDD had
an average seek time of about 600 ms;. Some early PC drives
used a stepper motor to move the heads, and as a result had seek times
as slow as 80–120 ms, but this was quickly improved by voice
coil type actuation in the 1980s, reducing seek times to around
Seek time has continued to improve slowly over time. The
fastest server drives today have a seek time around 4 ms. The
average seek time is strictly the time to do all possible seeks
divided by the number of all possible seeks, but in practice is
determined by statistical methods or simply approximated as the time
of a seek over one-third of the number of tracks.
Rotational latency is incurred because the desired disk sector may not
be directly under the head when data transfer is requested. Average
rotational latency is shown in the table, based on the statistical
relation that the average latency is one-half the rotational period.
The bit rate or data transfer rate (once the head is in the right
position) creates delay which is a function of the number of blocks
transferred; typically relatively small, but can be quite long with
the transfer of large contiguous files.
Delay may also occur if the drive disks are stopped to save energy.
Defragmentation is a procedure used to minimize delay in retrieving
data by moving related items to physically proximate areas on the
disk. Some computer operating systems perform defragmentation
automatically. Although automatic defragmentation is intended to
reduce access delays, performance will be temporarily reduced while
the procedure is in progress.
Time to access data can be improved by increasing rotational speed
(thus reducing latency) or by reducing the time spent seeking.
Increasing areal density increases throughput by increasing data rate
and by increasing the amount of data under a set of heads, thereby
potentially reducing seek activity for a given amount of data. The
time to access data has not kept up with throughput increases, which
themselves have not kept up with growth in bit density and storage
Average rotational latency
Data transfer rate
As of 2010[update], a typical 7,200-rpm desktop HDD has a sustained
"disk-to-buffer" data transfer rate up to 1,030 Mbit/s. This
rate depends on the track location; the rate is higher for data on the
outer tracks (where there are more data sectors per rotation) and
lower toward the inner tracks (where there are fewer data sectors per
rotation); and is generally somewhat higher for 10,000-rpm drives. A
current widely used standard for the "buffer-to-computer" interface is
Gbit/s SATA, which can send about 300 megabyte/s (10-bit
encoding) from the buffer to the computer, and thus is still
comfortably ahead of today's disk-to-buffer transfer rates. Data
transfer rate (read/write) can be measured by writing a large file to
disk using special file generator tools, then reading back the file.
Transfer rate can be influenced by file system fragmentation and the
layout of the files.
HDD data transfer rate depends upon the rotational speed of the
platters and the data recording density. Because heat and vibration
limit rotational speed, advancing density becomes the main method to
improve sequential transfer rates. Higher speeds require a more
powerful spindle motor, which creates more heat. While areal density
advances by increasing both the number of tracks across the disk and
the number of sectors per track, only the latter increases the data
transfer rate for a given rpm. Since data transfer rate performance
tracks only one of the two components of areal density, its
performance improves at a lower rate.
Other performance considerations include quality-adjusted price, power
consumption, audible noise, and both operating and non-operating shock
Federal Reserve Board
Federal Reserve Board has a quality-adjusted price index for
large-scale enterprise storage systems including three or more
enterprise HDDs and associated controllers, racks and cables. Prices
for these large-scale storage systems improved at the rate of ‒30%
per year during 2004–2009 and ‒22% per year during
Access and interfaces
Main article: hard disk drive interface
Inner view of a 1998 Seagate HDD that used
Parallel ATA interface
SATA drive on top of a 3.5-inch
SATA drive, close-up of data
and power connectors
Current hard drives connect to a computer over one of several bus
types, including parallel ATA,
Serial ATA , SCSI, Serial Attached SCSI
(SAS), and Fibre Channel. Some drives, especially external portable
drives, use IEEE 1394, or USB. All of these interfaces are
digital; electronics on the drive process the analog signals from the
read/write heads. Current drives present a consistent interface to the
rest of the computer, independent of the data encoding scheme used
internally, and independent of the physical number of disks and heads
within the drive.
Typically a DSP in the electronics inside the drive takes the raw
analog voltages from the read head and uses PRML and Reed–Solomon
error correction to decode the data, then sends that data out the
standard interface. That DSP also watches the error rate detected by
error detection and correction, and performs bad sector remapping,
data collection for Self-Monitoring, Analysis, and Reporting
Technology, and other internal tasks.
Modern interfaces connect the drive to the host interface with a
single data/control cable. Each drive also has an additional power
cable, usually direct to the power supply unit. Older interfaces had
separate cables for data signals and for drive control signals.
Small Computer System Interface
Small Computer System Interface (SCSI), originally named SASI for
Shugart Associates System Interface, was standard on servers,
workstations, Commodore Amiga,
Atari ST and
Apple Macintosh computers
through the mid-1990s, by which time most models had been transitioned
to IDE (and later, SATA) family disks. The length limit of the data
cable allows for external
Integrated Drive Electronics
Integrated Drive Electronics (IDE), later standardized under the name
AT Attachment (ATA, with the alias PATA (Parallel ATA) retroactively
added upon introduction of SATA) moved the HDD controller from the
interface card to the disk drive. This helped to standardize the
host/controller interface, reduce the programming complexity in the
host device driver, and reduced system cost and complexity. The 40-pin
IDE/ATA connection transfers 16 bits of data at a time on the
data cable. The data cable was originally 40-conductor, but later
higher speed requirements led to an "ultra DMA" (UDMA) mode using an
80-conductor cable with additional wires to reduce cross talk at high
EIDE was an unofficial update (by Western Digital) to the original IDE
standard, with the key improvement being the use of direct memory
access (DMA) to transfer data between the disk and the computer
without the involvement of the CPU, an improvement later adopted by
the official ATA standards. By directly transferring data between
memory and disk, DMA eliminates the need for the CPU to copy byte per
byte, therefore allowing it to process other tasks while the data
Fibre Channel (FC) is a successor to parallel
SCSI interface on
enterprise market. It is a serial protocol. In disk drives usually the
Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC
has much broader usage than mere disk interfaces, and it is the
cornerstone of storage area networks (SANs). Recently other protocols
for this field, like i
ATA over Ethernet have been developed
as well. Confusingly, drives usually use copper twisted-pair cables
for Fibre Channel, not fibre optics. The latter are traditionally
reserved for larger devices, such as servers or disk array
Serial Attached SCSI
Serial Attached SCSI (SAS). The SAS is a new generation serial
communication protocol for devices designed to allow for much higher
speed data transfers and is compatible with SATA. SAS uses a
mechanically identical data and power connector to standard 3.5-inch
SATA1/SATA2 HDDs, and many server-oriented SAS
RAID controllers are
also capable of addressing
SATA HDDs. SAS uses serial communication
instead of the parallel method found in traditional
SCSI devices but
Serial ATA (SATA). The
SATA data cable has one data pair for
differential transmission of data to the device, and one pair for
differential receiving from the device, just like EIA-422. That
requires that data be transmitted serially. A similar differential
signaling system is used in RS485, LocalTalk, USB, FireWire, and
Integrity and failure
Close-up of an HDD head resting on a disk platter; its mirror
reflection is visible on the platter surface.
Main articles: hard disk drive failure and Data recovery
Solid-state drive § SSD reliability and failure modes
Due to the extremely close spacing between the heads and the disk
surface, HDDs are vulnerable to being damaged by a head crash – a
failure of the disk in which the head scrapes across the platter
surface, often grinding away the thin magnetic film and causing data
loss. Head crashes can be caused by electronic failure, a sudden power
failure, physical shock, contamination of the drive's internal
enclosure, wear and tear, corrosion, or poorly manufactured platters
The HDD's spindle system relies on air density inside the disk
enclosure to support the heads at their proper flying height while the
disk rotates. HDDs require a certain range of air densities to operate
properly. The connection to the external environment and density
occurs through a small hole in the enclosure (about 0.5 mm in
breadth), usually with a filter on the inside (the breather
filter). If the air density is too low, then there is not enough
lift for the flying head, so the head gets too close to the disk, and
there is a risk of head crashes and data loss. Specially manufactured
sealed and pressurized disks are needed for reliable high-altitude
operation, above about 3,000 m (9,800 ft). Modern disks
include temperature sensors and adjust their operation to the
operating environment. Breather holes can be seen on all disk drives
– they usually have a sticker next to them, warning the user not to
cover the holes. The air inside the operating drive is constantly
moving too, being swept in motion by friction with the spinning
platters. This air passes through an internal recirculation (or
"recirc") filter to remove any leftover contaminants from manufacture,
any particles or chemicals that may have somehow entered the
enclosure, and any particles or outgassing generated internally in
normal operation. Very high humidity present for extended periods of
time can corrode the heads and platters.
For giant magnetoresistive (GMR) heads in particular, a minor head
crash from contamination (that does not remove the magnetic surface of
the disk) still results in the head temporarily overheating, due to
friction with the disk surface, and can render the data unreadable for
a short period until the head temperature stabilizes (so called
"thermal asperity", a problem which can partially be dealt with by
proper electronic filtering of the read signal).
When the logic board of a hard disk fails, the drive can often be
restored to functioning order and the data recovered by replacing the
circuit board with one of an identical hard disk. In the case of
read-write head faults, they can be replaced using specialized tools
in a dust-free environment. If the disk platters are undamaged, they
can be transferred into an identical enclosure and the data can be
copied or cloned onto a new drive. In the event of disk-platter
failures, disassembly and imaging of the disk platters may be
required. For logical damage to file systems, a variety of tools,
including fsck on
UNIX-like systems and
CHKDSK on Windows, can be used
for data recovery. Recovery from logical damage can require file
A common expectation is that hard disk drives designed and marketed
for server use will fail less frequently than consumer-grade drives
usually used in desktop computers. However, two independent studies by
Carnegie Mellon University and Google found that the "grade"
of a drive does not relate to the drive's failure rate.
A 2011 summary of research, into SSD and magnetic disk failure
Tom's Hardware summarized research findings as
Mean time between failures
Mean time between failures (MTBF) does not indicate reliability; the
annualized failure rate is higher and usually more relevant.
Magnetic disks do not have a specific tendency to fail during early
use, and temperature has only a minor effect; instead, failure rates
steadily increase with age.
S.M.A.R.T. warns of mechanical issues but not other issues affecting
reliability, and is therefore not a reliable indicator of
Failure rates of drives sold as "enterprise" and "consumer" are "very
much similar", although these drive types are customized for their
different operating environments.
In drive arrays, one drive's failure significantly increases the
short-term risk of a second drive failing.
They typically store between 60 GB and 4 TB and rotate at
5,400 to 10,000 rpm, and have a media transfer rate of
Gbit/s or higher (1 GB = 109 bytes; 1
Gbit/s = 109
bit/s). As of February 2017, the highest-capacity desktop HDDs stored
12 TB, with plans to release a 14TB one in later 2017.
As of 2016, the typical speed of a hard drive in an average desktop
computer is 7200 RPM, whereas low-cost desktop computers may use 5900
RPM or 5400 RPM drives. For some time in the 2000s and early 2010s
some desktop users would also use 10k RPM drives such as Western
Digital Raptor but such drives have become much rarer as of 2016 and
are not commonly used now, having been replaced by NAND flash-based
Mobile (laptop) HDDs
SATA 2.5-inch 10,000 rpm HDDs,
factory-mounted in 3.5-inch adapter frames
Smaller than their desktop and enterprise counterparts, they tend to
be slower and have lower capacity. Mobile HDDs spin at 4,200 rpm,
5,200 rpm, 5,400 rpm, or 7,200 rpm, with 5,400 rpm
being typical. 7,200 rpm drives tend to be more expensive and
have smaller capacities, while 4,200 rpm models usually have very
high storage capacities. Because of smaller platter(s), mobile HDDs
generally have lower capacity than their desktop counterparts.
There are also 2.5-inch drives spinning at 10,000 rpm, which
belong to the enterprise segment with no intention to be used in
Typically used with multiple-user computers running enterprise
software. Examples are: transaction processing databases, internet
infrastructure (email, webserver, e-commerce), scientific computing
software, and nearline storage management software. Enterprise drives
commonly operate continuously ("24/7") in demanding environments while
delivering the highest possible performance without sacrificing
reliability. Maximum capacity is not the primary goal, and as a result
the drives are often offered in capacities that are relatively low in
relation to their cost.
The fastest enterprise HDDs spin at 10,000 or 15,000 rpm, and can
achieve sequential media transfer speeds above 1.6 Gbit/s
and a sustained transfer rate up to 1 Gbit/s. Drives running
at 10,000 or 15,000 rpm use smaller platters to mitigate
increased power requirements (as they have less air drag) and
therefore generally have lower capacity than the highest capacity
desktop drives. Enterprise HDDs are commonly connected through Serial
SCSI (SAS) or
Fibre Channel (FC). Some support multiple
ports, so they can be connected to a redundant host bus adapter.
Enterprise HDDs can have sector sizes larger than 512 bytes (often
520, 524, 528 or 536 bytes). The additional per-sector space can be
used by hardware
RAID controllers or applications for storing Data
Integrity Field (DIF) or Data Integrity Extensions (DIX) data,
resulting in higher reliability and prevention of silent data
Consumer electronics HDDs
They include drives embedded into digital video recorders and
automotive vehicles. The former are configured to provide a guaranteed
streaming capacity, even in the face of read and write errors, while
the latter are built to resist larger amounts of shock. They usually
spin at a speed of 5400 RPM.
Manufacturers and sales
Diagram of HDD manufacturer consolidation
See also: history of hard disk drives and list of defunct hard disk
More than 200 companies have manufactured HDDs over time. But
consolidations have concentrated production into just three
manufacturers today: Western Digital, Seagate, and Toshiba.
Worldwide revenue for disk storage declined 4% per year, from a peak
of $38 billion in 2012 to $27 billion in 2016. Production of HDDs grew
16% per year, from 335 exabytes in 2011 to 693 exabytes in 2016.
Shipments declined 7% per year during this time period, from 620
million units to 425 million. Seagate and
Western Digital each
have 40–45% of unit shipments, while
Toshiba has 13–17%. The
average sales price for the two largest manufacturers was $60 per unit
Competition from solid-state drives
The maximum areal storage density for flash memory used in solid state
drives (SSDs) is 2.8 Tbit/in2 in laboratory demonstrations as of
2016, and the maximum for HDDs is 1.5 Tbit/in2. The areal density
of flash memory is doubling every two years, similar to Moore's law
(40% per year) and faster than the 10–20% per year for HDDs. As of
2016, maximum capacity was 10 terabytes for an HDD, and
15 terabytes for an SSD. HDDs were used in 70% of the desktop
and notebook computers produced in 2016, and SSDs were used in 30%.
The usage share of HDDs is declining and could drop below 50% in
2018–2019 according to one forecast, because SSDs are replacing
smaller-capacity (less than one-terabyte) HDDs in desktop and notebook
computers and MP3 players.
The market for silicon-based flash memory (NAND) chips, used in SSDs
and other applications, is growing rapidly. Worldwide revenue grew 12%
per year during 2011–2016. It rose from $22 billion in 2011 to $39
billion in 2016, while production grew 46% per year from 19 exabytes
to 120 exabytes.
External hard disk drives
USB mass storage device and disk enclosure
Toshiba 1 TB 2.5" external
USB 2.0 hard disk drive
External hard disk drives typically connect via USB; variants using
USB 2.0 interface generally have slower data transfer rates when
compared to internally mounted hard drives connected through SATA.
Plug and play
Plug and play drive functionality offers system compatibility and
features large storage options and portable design. As of
March 2015[update], available capacities for external hard disk
drives ranged from 500 GB to 10 TB.
External hard disk drives are usually available as pre-assembled
integrated products, but may be also assembled by combining an
external enclosure (with
USB or other interface) with a separately
purchased drive. They are available in 2.5-inch and 3.5-inch sizes;
2.5-inch variants are typically called portable external drives, while
3.5-inch variants are referred to as desktop external drives.
"Portable" drives are packaged in smaller and lighter enclosures than
the "desktop" drives; additionally, "portable" drives use power
provided by the
USB connection, while "desktop" drives require
external power bricks.
Features such as biometric security or multiple interfaces (for
example, Firewire) are available at a higher cost. There are
pre-assembled external hard disk drives that, when taken out from
their enclosures, cannot be used internally in a laptop or desktop
computer due to embedded
USB interface on their printed circuit
boards, and lack of
SATA (or Parallel ATA) interfaces.
In GUIs, hard disk drives are commonly symbolized with a drive icon
Information Technology portal
Automatic acoustic management
Click of death
Error recovery control
Network drive (file server, shared resource)
^ This is the original filing date of the application which led to US
Patent 3,503,060, generally accepted as the definitive disk drive
^ Further inequivalent terms used to describe various hard disk drives
include disk drive, disk file, direct access storage device (DASD),
CKD disk, and Winchester disk drive (after the
IBM 3340). The term
"DASD" includes other devices beside disks.
^ Comparable in size to a large side-by-side refrigerator.
^ The 1.8-inch form factor is obsolete; sizes smaller than 2.5 inches
have been replaced by flash memory.
^ Initially gamma iron oxide particles in an epoxy binder, the
recording layer in a modern HDD typically is domains of a granular
Cobalt-Chrome-Platinum-based alloy physically isolated by an oxide to
enable perpendicular recording.
^ Historically a variety of run-length limited codes have been used in
magnetic recording including for example, codes named FM, MFM and GCR
which are no longer used in modern HDDs.
^ a b Expressed using decimal multiples.
^ a b Expressed using binary multiples.
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^ 600 divided by 2.5 .
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transducer (NFT) and a number of other components not used or mass
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Find more aboutHard disk driveat's sister projects
Definitions from Wiktionary
Media from Wikimedia Commons
News from Wikinews
Textbooks from Wikibooks
Learning resources from Wikiversity
Hard Disk Drives Encyclopedia
Video showing an opened HD working
Average seek time of a computer disk
Timeline: 50 Years of Hard Drives
HDD from inside: Tracks and Zones. How hard it can be?
Hard disk hacking – firmware modifications, in eight parts,
going as far as booting a Linux kernel on an ordinary HDD controller
Hiding Data in Hard Drive’s Service Areas, February 14, 2013, by
Rotary Acceleration Feed Forward (RAFF) Information Sheet, Western
Digital, January 2013
PowerChoice Technology for Hard Disk Drive Power Savings and
Flexibility, Seagate Technology, March 2010
Shingled Magnetic Recording (SMR), HGST, Inc., 2015
The Road to Helium, HGST, Inc., 2015
Research paper about perspective usage of magnetic photoconductors in
magneto-optical data storage.
Magnetic storage media
Ferrite core (1949)
Hard disk (1956)
Stripe card (1956)
Thin film (1962)
Floppy disk (1969)
Hard disk drive
Hard disk drive manufacturers
History of hard disk drives
Computer Memories Inc.
Control Data Corporation
Digital Equipment Corporation
Storage Technology Corporation
Basic computer components
Refreshable braille display
Refreshable braille display
USB flash drive
Central processing unit
Central processing unit (CPU)
HDD / SSD / SSHD
Network interface controller
Random-access memory (RAM)
FireWire (IEEE 1394)
HDMI / DVI / VGA
Disk image file formats
Comparison of disc image software
IMG, IMA, IMZ
Disc Description Protocol
Convention: Any item in this table that has the form of "A+B" or
"A+B+C" indicates a disk format that spans multiple files, where A
contains the bulk of the data, and B and C are sidecar files.