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 variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

s, specifically a USB Device Class, defined by the USB Implementers Forum that makes a USB 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 sets 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 hard disk drive platter, pla ...

s * External optical drives, including CD and DVD reader and writer drives *

USB flash drive A flash drive (also thumb drive, memory stick, and pen drive/pendrive) is a data storage device that includes flash memory with an integrated USB interface. A typical USB drive is removable, rewritable, and smaller than an optical disc, and u ...

s * Solid-state drives * Adapters between standard flash memory cards and USB connections *

Digital camera A digital camera, also called a digicam, is a camera that captures photographs in Digital data storage, digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film or film stock. Dig ...

s * Portable media players * Card readers * PDAs * Mobile phones Devices supporting this standard are known as MSC (Mass Storage Class) devices. While MSC is the original abbreviation, UMS (Universal Mass Storage) has also become common use.

Operating system support

Most mainstream

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

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 and Windows NT 4.0. Windows 95 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. It was the second operating system in the 9x line, as the successor to Windows 95. It was Software ...

), 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, targeting the server and business markets. It is the direct successor to Windows NT 4.0, and was Software release life cycle#Release to manufacturing (RT ...

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

Windows Me Windows Me (Millennium Edition) is an operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. It was the successor to Windows 98, and was released to manufacturing on June 19, 2000, and t ...

and all later Windows versions also include support.

Windows Mobile Windows Mobile is a discontinued mobile operating system developed by Microsoft for smartphones and personal digital assistants (PDA). Designed to be the portable equivalent of the Windows desktop OS in the emerging Mobile device, mobile/port ...

supports accessing most USB mass-storage devices formatted with FAT on devices with USB Host. However, portable devices typically cannot provide enough power for hard-drive disk enclosures (a hard drive typically requires the maximum 2.5 W in the USB specification) without a self-powered USB hub. 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-system issues; see device access, 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 malware that, when executed, replicates itself by modifying other computer programs and Code injection, inserting its own Computer language, code into those programs. If this replication succeeds, the affected areas ...

es. Beginning with Windows 7, 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 op ...

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

Classic Mac OS and macOS

Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

's Mac OS 9 and

macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...

support USB mass storage; Mac OS 8.6 supported USB mass storage through an optional driver.

Linux

The

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

has supported USB mass-storage devices since version 2.3.47 (2001, backported to kernel 2.2.18). This support includes quirks and silicon/firmware bug workarounds as well as additional functionality for devices and controllers (vendor-enabled functions such as ATA command pass-through for ATA-USB bridges, used for S.M.A.R.T. or temperature monitoring, controlling the spin-up and spin-down of hard disk drives, and other options). Mobile devices running Android 6 or higher also supports USB mass storage through dual-role USB on USB-C ports, and USB-OTG on older ports.

Other Unix-related systems

Solaris has supported devices since its version 2.8 (1998),

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

since its version 1.5 (2000), FreeBSD since its version 4.0 (2000) and OpenBSD since its version 2.7 (2000). Digital UNIX (later known as

Tru64 UNIX Tru64 UNIX is a discontinued 64-bit UNIX operating system for the DEC Alpha, Alpha instruction set architecture (ISA), currently owned by Hewlett-Packard (HP). Previously, Tru64 UNIX was a product of Compaq, and before that, Digital Equipment Corp ...

) has supported USB and USB mass-storage devices since its version 4.0E (1998). AIX 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 Xbox (console), original Xbox, it is the second console in the Xbox#Consoles, Xbox series. It was officially unveiled on MTV on May 12, 2005, with detail ...

and

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

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 and TI-84 Plus Silver Edition 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 (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 (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 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 such as

s, cameras, or digital audio players is Microsoft's FAT or FAT32 file system (with optional support for long filenames). However, USB mass storage devices may be formatted with any other file system, such as NTFS on Windows NT, HFS Plus on

ext2 ext2, or second extended file system, is a file system for the Linux kernel (operating system), kernel. It was initially designed by French software developer Rémy Card as a replacement for the extended file system (ext). Having been designed ...

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

, or Unix File System on Solaris 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) 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). Some devices have a write-protection switch (or option) allowing them to be used in read-only mode. 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 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 family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and Mobile broadband modem, mobile broadba ...

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 hard disk drive platter, pla ...

commands, such as Tagged Command Queuing 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 driv ...

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