The USB mass storage device class (also known as USB MSC or UMS) is a set of computing

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchr ...

s, specifically a USB Device Class, defined by the

USB Implementers Forum The USB Implementers Forum (USB-IF) is a nonprofit organization created to promote and support USB (Universal Serial Bus). Its main activities are the promotion and marketing of USB, Wireless USB, USB On-The-Go, and the maintenance of the spe ...

that makes a

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

device accessible to a host computing device and enables file transfers between the host and the USB device. To a host, the USB device acts as an external hard drive; the protocol set interfaces with a number of storage devices.

Uses

Devices connected to computers via this standard include: * External magnetic

hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...

s * External optical drives, including CD and

DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kind ...

reader and writer drives *

USB flash drive A USB flash drive (also called a thumb drive) is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than . Since firs ...

s *

Solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is ...

s * Adapters between standard flash

memory card A memory card is an electronic data storage device used for storing digital information, typically using flash memory. These are commonly used in digital portable electronic devices. They allow adding memory to such devices using a card in a so ...

s and USB connections *

Digital camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile device ...

s *

Portable media player A portable media player (PMP) (also including the related digital audio player (DAP)) is a portable consumer electronics device capable of storing and playing digital media such as audio, images, and video files. The data is typically stored o ...

s *

Card reader A card reader is a data input device that reads data from a card-shaped storage medium. The first were punched card readers, which read the paper or cardboard punched cards that were used during the first several decades of the computer industry ...

s *

PDA PDA may refer to: Science and technology * Patron-driven acquisition, a mechanism for libraries to purchase books *Personal digital assistant, a mobile device * Photodiode array, a type of detector * Polydiacetylenes, a family of conducting poly ...

s *

Mobile phones A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...

Devices supporting this standard are known as MSC (Mass Storage Class) devices. While MSC is the original abbreviation, UMS (Universal Mass Storage) has also come into common use.

Operating system support

Most mainstream

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...

s include support for USB mass storage devices; support on older systems is usually available through patches.

Microsoft Windows

Microsoft Windows has supported MSC since Windows 2000. There is no support for USB supplied by Microsoft in Windows before

Windows 95 Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturin ...

and

Windows NT 4.0 Windows NT 4.0 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It is the direct successor to Windows NT 3.51, which was released to manufacturing on July 31, 1996, and then to retail ...

OSR2.1, an update to the operating system, featured limited support for USB. During that time no generic USB mass-storage driver was produced by Microsoft (including for

Windows 98 Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released to ...

), and a device-specific driver was needed for each type of USB storage device. Third-party, freeware drivers became available for Windows 98 and Windows 98SE, and third-party drivers are also available for Windows NT 4.0.

Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was Software release life cycle#Release to manufacturing (RTM), releas ...

has support (via a generic driver) for standard USB mass-storage devices;

Windows Me Windows Millennium Edition, or Windows Me (marketed with the pronunciation of the pronoun "me"), is an operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. It is the successor to Windo ...

and all later Windows versions also include support.

Windows Mobile Windows Mobile is a discontinued family of mobile operating systems developed by Microsoft for smartphones and personal digital assistants. Its origin dated back to Windows CE in 1996, though Windows Mobile itself first appeared in 2000 as Pock ...

supports accessing most USB mass-storage devices formatted with

FAT In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers spec ...

on devices with USB Host. However, portable devices typically cannot provide enough power for hard-drive

disk enclosure A disk enclosure is a specialized casing designed to hold and power disk drives while providing a mechanism to allow them to communicate to one or more separate computers. Drive enclosures provide power to the drives therein and convert the dat ...

s (a hard drive typically requires the maximum 2.5 W in the USB specification) without a self-powered

USB hub Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad v ...

. A Windows Mobile device cannot display its file system as a mass-storage device unless the device implementer adds that functionality. However, third-party applications add MSC emulation to most WM devices (commercial Softick CardExport and free WM5torage). Only memory cards (not internal-storage memory) can generally be exported, due to file-systems issues; see

device access A device is usually a constructed tool. Device may also refer to: Technology Computing * Device, a colloquial term encompassing desktops, laptops, tablets, smartphones, etc. * Device file, an interface of a device driver * Peripheral, any devic ...

, below. The AutoRun feature of Windows worked on all removable media, allowing USB storage devices to become a portal for

computer virus A computer virus is a type of computer program that, when executed, replicates itself by modifying other computer programs and inserting its own code. If this replication succeeds, the affected areas are then said to be "infected" with a compu ...

es. Beginning with

Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was released to manufacturing on July 22, 2009, and became generally available on October 22, 2009. It is the successor to Windows Vista, released nearly ...

, Microsoft limited AutoRun to CD and DVD drives, updating previous Windows versions.

MS-DOS

Neither

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...

nor most compatible operating systems included support for USB. Third-party generic drivers, such as Duse, USBASPI and DOSUSB, are available to support USB mass-storage devices.

FreeDOS FreeDOS (formerly Free-DOS and PD-DOS) is a free software operating system for IBM PC compatible computers. It intends to provide a complete MS-DOS-compatible environment for running Legacy system, legacy software and supporting embedded system ...

supports USB mass storage as an Advanced SCSI Programming Interface (ASPI) interface.

Classic Mac OS and macOS

Apple Computer Apple Inc. is an American multinational technology company headquartered in Cupertino, California, United States. Apple is the largest technology company by revenue (totaling in 2021) and, as of June 2022, is the world's biggest company b ...

Mac OS 9 Mac OS 9 is the ninth major release of Apple Inc., Apple's classic Mac OS operating system which was succeeded by macOS, Mac OS X (renamed to OS X in 2011 and macOS in 2016) in 2001. Introduced on October 23, 1999, it was promoted by Apple as "T ...

and

macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...

support USB mass storage; Mac OS 8.5.1 supported USB mass storage through an optional driver.

Linux

The

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...

has supported USB mass-storage devices since its 2.4 series (2001), and a

backport Backporting is the action of taking parts from a newer version of a software system or software component and porting them to an older version of the same software. It forms part of the maintenance step in a software development process, and it is ...

to kernel 2.2.18 has been made. In Linux, more features exist in addition to the generic drivers for USB mass-storage device class devices, including quirks, bug fixes and additional functionality for devices and controllers (vendor-enabled functions such as ATA command pass-through for ATA-USB bridges, which is useful for S.M.A.R.T. or temperature monitoring, controlling the spin-up and spin-down of hard disk drives, and other options). This includes a certain portion of Android-based devices, through support of

USB-OTG USB On-The-Go (USB OTG or just OTG) is a specification first used in late 2001 that allows USB devices, such as tablets or smartphones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, mouse or keyboar ...

, since Android uses the Linux kernel.

Other Unix-related systems

Solaris Solaris may refer to: Arts and entertainment Literature, television and film * ''Solaris'' (novel), a 1961 science fiction novel by Stanisław Lem ** ''Solaris'' (1968 film), directed by Boris Nirenburg ** ''Solaris'' (1972 film), directed by ...

has supported devices since its version 2.8 (1998),

NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...

since its version 1.5 (2000),

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

since its version 4.0 (2000) and

OpenBSD OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD 1.0. According to the website, the OpenBSD project em ...

since its version 2.7 (2000). Digital UNIX (later known as Tru64 UNIX), has supported USB and USB mass-storage devices since its version 4.0E (1998).

AIX Aix or AIX may refer to: Computing * AIX, a line of IBM computer operating systems *An Alternate Index, for a Virtual Storage Access Method Key Sequenced Data Set * Athens Internet Exchange, a European Internet exchange point Places Belgi ...

has supported USB mass-storage devices since its 5.3 T9 and 6.1 T3 versions; however, it is not well-supported and lacks features such as partitioning and general blocking.

Game consoles and embedded devices

The

Xbox 360 The Xbox 360 is a home video game console developed by Microsoft. As the successor to the original Xbox, it is the second console in the Xbox series. It competed with Sony's PlayStation 3 and Nintendo's Wii as part of the seventh generation ...

and

PlayStation 3 The PlayStation 3 (PS3) is a home video game console developed by Sony Interactive Entertainment, Sony Computer Entertainment. The successor to the PlayStation 2, it is part of the PlayStation brand of consoles. It was first released on Novemb ...

support most mass-storage devices for the data transfer of media such as pictures and music. As of April 2010, the Xbox 360 (a) used a mass-storage device for saved games and the PS3 allowed transfers between devices on a mass-storage device. Independent developers have released drivers for the

TI-84 Plus The TI-84 Plus is a graphing calculator made by Texas Instruments which was released in early 2004. There is no original TI-84, only the TI-84 Plus, the TI-84 Plus Silver Edition models, and the TI-84 Plus CE. The TI-84 Plus is an enhanced ve ...

and

TI-84 Plus Silver Edition The TI-84 Plus is a graphing calculator made by Texas Instruments which was released in early 2004. There is no original TI-84, only the TI-84 Plus, the TI-84 Plus Silver Edition models, and the TI-84 Plus CE. The TI-84 Plus is an enhanced ve ...

to access USB mass-storage devices. In these calculators, the usb8x driver supports the msd8x user-interface application.

Device access

The USB mass-storage specification provides an interface to a number of industry-standard command sets, allowing a device to disclose its subclass. In practice, there is little support for specifying a command set via its subclass; most drivers only support the SCSI transparent command set, designating their subset of the SCSI command set with their

SCSI Peripheral Device Type A SCSI Peripheral Device Type describes the capabilities provided by a SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI st ...

(PDT). Subclass codes specify the following command sets: # Reduced Block Commands (RBC) # SFF-8020i, MMC-2 (used by ATAPI-style CD and DVD drives) # QIC-157 (tape drives) #

Uniform Floppy Interface A uniform is a variety of clothing worn by members of an organization while participating in that organization's activity. Modern uniforms are most often worn by armed forces and paramilitary organizations such as police, emergency services, ...

(UFI) # SFF-8070i (used by ARMD-style devices) # SCSI transparent command set (use "inquiry" to obtain the PDT) The specification does not require a particular

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

on conforming devices. Based on the specified command set and any subset, it provides a means to read and write sectors of data (similar to the low-level interface used to access a

). Operating systems may treat a USB mass-storage device like a hard drive; users may partition it in any format (such as MBR and GPT), and format it with any file system. Because of its relative simplicity, the most common file system on

embedded devices An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...

such as

s, cameras, or digital audio players is Microsoft's

FAT32 File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by c ...

file system (with optional support for

long filename Long filename (LFN) support is Microsoft's backward-compatible extension of the 8.3 filename (short filename) naming scheme used in DOS. Long filenames can be more descriptive, including longer filename extensions such as .jpeg, .tiff, .html, a ...

s). Large, USB-based hard disks may be formatted with

NTFS New Technology File System (NTFS) is a proprietary journaling file system developed by Microsoft. Starting with Windows NT 3.1, it is the default file system of the Windows NT family. It superseded File Allocation Table (FAT) as the preferred fil ...

, which (except for Windows) is less supported. However, a

keydrive A USB flash drive (also called a thumb drive) is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than . Since firs ...

or other device may be formatted with another file system (

HFS Plus HFS Plus or HFS+ (also known as Mac OS Extended or HFS Extended) is a journaling file system developed by Apple Inc. It replaced the Hierarchical File System (HFS) as the primary file system of Apple computers with the 1998 release of Mac OS ...

on an

Apple Macintosh The Mac (known as Macintosh until 1999) is a family of personal computers designed and marketed by Apple Inc. Macs are known for their ease of use and minimalist designs, and are popular among students, creative professionals, and software en ...

, or

Ext2 The ext2 or second extended file system is a file system for the Linux kernel. It was initially designed by French software developer Rémy Card as a replacement for the extended file system (ext). Having been designed according to the same pr ...

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...

, or

Unix File System The Unix file system (UFS) is a family of file systems supported by many Unix and Unix-like operating systems. It is a distant descendant of the original filesystem used by Version 7 Unix. Design A UFS volume is composed of the following p ...

or BSD). This choice may limit (or prevent) access to a device's contents by equipment using a different operating system. OS-dependent storage options include LVM, partition tables and software encryption. In cameras, MP3 players and similar devices which must access a file system independent of an external host, the FAT32 file system is preferred by manufacturers. All such devices halt their file-system (

dismount Gymnastics is a type of sport that includes physical exercises requiring balance, strength, flexibility, agility, coordination, dedication and endurance. The movements involved in gymnastics contribute to the development of the arms, legs, sho ...

) before making it available to a host operating system to prevent file-system corruption or other damage (although it is theoretically possible for both devices to use read-only mode or a

cluster file system A clustered file system is a file system which is shared by being simultaneously mounted on multiple servers. There are several approaches to clustering, most of which do not employ a clustered file system (only direct attached storage for ...

). Some devices have a write-protection switch (or option) allowing them to be used in read-only mode; this makes files available for shared use without the risk of virus infection. Two main partitioning schemes are used by vendors of pre-formatted devices. One puts the file system (usually FAT32) directly on the device without partitioning, making it start from sector 0 without additional boot sectors, headers or partitions. The other uses a DOS partition table (and MBR code), with one partition spanning the entire device. This partition is often aligned to a high power of two of the sectors (such as 1 or 2 MB), common in

solid state drives A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is ...

for performance and durability. Some devices with embedded storage resembling a USB mass-storage device (such as MP3 players with a USB port) will report a damaged (or missing) file system if they are reformatted with a different file system. However, most default-partition devices may be repartitioned (by reducing the first partition and file system) with additional partitions. Such devices will use the first partition for their own operations; after connecting to the host system, all partitions are available. Devices connected by a single USB port may function as multiple USB devices, one of which is a USB mass-storage device. This simplifies distribution and access to drivers and documentation, primarily for the Microsoft Windows and Mac OS X operating systems. Such drivers are required to make full use of the device, usually because it does not fit a standard USB class or has additional functionality. An embedded USB mass-storage device makes it possible to install additional drivers without CD-ROM disks, floppies or Internet access to a vendor website; this is important, since many modern systems are supplied without optical or floppy drives. Internet access may be unavailable because the device provides network access (wireless,

GSM The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...

or Ethernet cards). The embedded USB mass storage is usually made permanently read-only by the vendor, preventing accidental corruption and use for other purposes (although it may be updated with proprietary protocols when performing a firmware upgrade). Advantages of this method of distribution are lower cost, simplified installation and ensuring driver portability.

Design

Some advanced

hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...

commands, such as

Tagged Command Queuing Tagged Command Queuing (TCQ) is a technology built into certain ATA and SCSIin the form of Parallel SCSI, Serial attached SCSI, and Fibre Channel drives hard drives. It allows the operating system to send multiple read and write requests to a ha ...

and

Native Command Queuing In computing, Native Command Queuing (NCQ) is an extension of the Serial ATA protocol allowing hard disk drives to internally optimize the order in which received read and write commands are executed. This can reduce the amount of unnecessary dri ...

(which may increase performance), ATA Secure Erase (which allows all data on the drive to be securely erased) and S.M.A.R.T. (accessing indicators of drive reliability) exist as extensions to low-level drive command sets such as

SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...

, ATA and ATAPI. These features may not work when the drives are placed in a

that supports a USB mass-storage interface. Some USB mass-storage interfaces are generic, providing basic read-write commands; although that works well for basic data transfers with devices containing hard drives, there is no simple way to send advanced, device-specific commands to such USB mass-storage devices (though, devices may create their own communication protocols over a standard USB control interface). The

USB Attached SCSI USB Attached SCSI (UAS) or USB Attached SCSI Protocol (UASP) is a computer protocol used to move data to and from USB storage devices such as hard drives (HDDs), solid-state drives (SSDs), and thumb drives. UAS depends on the USB protocol, and u ...

(UAS) protocol, introduced in USB 3.0, fixes several of these issues, including command queuing, command pipes for hardware requiring them, and power management. Specific USB 2.0 chipsets had proprietary methods of achieving SCSI pass-through, which could be used to read S.M.A.R.T. data from drives using tools such as smartctl (using the option followed by "chipset"). More recent USB storage chipsets support the

SCSI / ATA Translation SCSI / ATA Translation (SAT) is a set of standards developed by the T10 subcommittee, defining how to communicate with ATA devices through a SCSI application layer. The standard attempts to be consistent with the SCSI architectural model, the ...

(SAT) as a generic protocol for interacting with ATA (and SATA) devices. Using esoteric ATA or SCSI pass-through commands (such as secure-erase or password protection) when a drive is connected via a USB bridge may cause drive failure, especially with the

hdparm hdparm is a command line program for Linux to set and view ATA hard disk drive hardware parameters and test performance. It can set parameters such as drive caches, sleep mode, power management, acoustic management, and DMA settings. GParted ...

utility.

References

External links

USB Mass Storage Device source code in FreeBSD

Linux kernel internals {{Computer-bus Computer storage buses Computer storage devices Mass storage device class