Editing USB mass storage device class

{{Short description|USB device class for drives}}
{{More citations needed|date=November 2022}}
[[File:USB flash drive.jpg|thumb|upright|alt=A USB thumb drive and its cap, next to a 100 millimeter ruler for scale|[[USB flash drive]]s typically implement the USB mass storage device class.]]

The '''USB mass storage device class''' (also known as '''USB MSC''' or '''UMS''') is a set of computing [[communications protocol]]s, specifically a [[USB#Device_classes|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 ==
[[File:Action camera being accessed via MSDC.jpg|thumb|An action camera being accessed via mass storage device class]]

Devices connected to computers via this standard include:
* External magnetic [[hard drive]]s
* External optical drives, including [[CD]] and [[DVD]] reader and writer drives
* [[USB flash drive]]s
* [[Solid-state drive]]s
* Adapters between standard flash [[memory card]]s and USB connections
* [[Digital camera]]s
* [[Portable media player]]s
* [[Card reader]]s
* [[Personal digital assistant|PDA]]s
* [[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]]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]]), 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]] has support (via a generic driver) for standard USB mass-storage devices; [[Windows Me]] and all later Windows versions also include support.

[[Windows Mobile]] supports accessing most USB mass-storage devices formatted with [[File Allocation Table|FAT]] on devices with USB Host. However, portable devices typically cannot provide enough power for hard-drive [[disk enclosure]]s (a {{convert|2.5|in|adj=on}} hard drive typically requires the maximum 2.5&nbsp;[[Watt|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|device access]], below.

The [[AutoRun]] feature of Windows worked on all removable media, allowing USB storage devices to become a portal for [[computer virus]]es. Beginning with [[Windows&nbsp;7]], Microsoft limited AutoRun to CD and DVD drives, updating previous Windows versions.<ref>{{cite web |url=http://blogs.technet.com/msrc/archive/2009/04/28/changes-in-windows-to-meet-changes-in-threat-landscape.aspx |title=Changes in Windows to Meet Changes in Threat Landscape |publisher=TechNet Blogs |date=2009-04-28 |access-date=2012-11-07}}</ref>

=== MS-DOS ===
Neither [[MS-DOS]] 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 Inc.|Apple]]'s [[Mac OS 9]] and [[macOS]] support USB mass storage; Mac OS 8.6 supported USB mass storage through an optional driver.<ref>{{Cite web|title=USB Mass Storage 1.3.5 - Macintosh Repository|url=https://www.macintoshrepository.org/161-usb-mass-storage-1-3-5|access-date=2024-11-27|website=www.macintoshrepository.org}}</ref>

=== Linux ===
The [[Linux kernel]] has supported USB mass-storage devices since version 2.3.47<ref>{{cite web|title=usb-storage.c - drivers/usb/usb-storage.c - Linux source code 2.3.47pre8 - Bootlin Elixir Cross Referencer|url=https://elixir.bootlin.com/linux/2.3.47pre8/source/drivers/usb/usb-storage.c|access-date=2024-11-27|website=elixir.bootlin.com}}</ref> (2001, [[backport|backported]] to kernel 2.2.18<ref>{{cite web|title=usb-storage.c - drivers/usb/usb-storage.c - Linux source code 2.2.18pre2 - Bootlin Elixir Cross Referencer|url=https://elixir.bootlin.com/linux/2.2.18pre2/source/drivers/usb/usb-storage.c|access-date=2024-11-27|website=elixir.bootlin.com}}</ref>). This support includes quirks and silicon/firmware bug workarounds as well as additional functionality for devices and controllers (vendor-enabled functions such as [[Parallel ATA|ATA]] command pass-through for ATA-USB bridges, used for [[Self-Monitoring, Analysis and Reporting Technology|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 (operating system)|Android]] 6 or higher<ref>{{cite web|title=Traditional storage {{!}} Android Open Source Project|at=sec. USB media support|url=https://source.android.com/docs/core/storage/traditional#support_usb_media|access-date=2024-11-27|website=source.android.com}}</ref> also supports USB mass storage through [[USB#Dual-role_USB_connections|dual-role USB]] on USB-C ports, and [[USB-OTG]] on older ports.

=== {{anchor|Other Unix-likes}}Other Unix-related systems ===
[[Solaris (operating system)|Solaris]] has supported devices since its version 2.8 (1998), [[NetBSD]] 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]]) 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.<ref>{{cite web|url=http://eserver.livejournal.com/27707.html |title=eserver: HOWTO: JFS2 on USB device on AIX 5.3.11.1 |website=Eserver.livejournal.com |date=2010-01-21 |access-date=2012-11-07 |url-status=dead |archive-url=https://web.archive.org/web/20120331082549/http://eserver.livejournal.com/27707.html |archive-date=2012-03-31 }}</ref>

=== Game consoles and embedded devices ===
The [[Xbox 360]] and [[PlayStation 3]] 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<ref>{{cite web|url=http://majornelson.com/archive/2010/03/26/USB-Memory-Support-for-the-Xbox-360-coming-April-6th.aspx |title=Xbox Live's Major Nelson » USB Memory Support for the Xbox 360 coming April 6th |website=Majornelson.com |date=2010-03-26 |access-date=2012-11-07}}</ref> 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.<ref>{{cite web|url=http://wikiti.brandonw.net/index.php?title=83Plus:Software:usb8x/Asm_Interface/MSD |title=83Plus:Software:usb8x/Asm Interface/MSD |publisher= WikiTI |date=2009-02-18 |access-date=2012-11-07}}</ref> In these calculators, the [[usb8x]] driver supports the [[msd8x]] user-interface application.

==Device access==
[[File:Usbcardreader.png|thumb|alt=A small, thin, gray box, with data card inserted in a bottom slot|USB [[card reader]]s typically implement the USB mass storage device class.]]
{{see also|USB Attached SCSI}}
{{missing information|section|Bulk-Only Transport: we keep linking to this article to contrast with UAS but there's no description here!|date=May 2023}}
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 command|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)
# [[Small Form Factor committee|SFF]]-8020i, [[MultiMedia Commands|MMC]]-2 (used by ATAPI-style CD and DVD drives)
# [[Quarter-inch cartridge|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 [[hard drive]]). 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 system|embedded devices]] such as [[USB flash drive]]s, cameras, or digital audio players is Microsoft's [[File Allocation Table|FAT]] or [[FAT32]] file system (with optional support for [[long filename]]s). However, USB mass storage devices may be formatted with any other file system, such as [[NTFS]] on Windows NT, [[HFS Plus]] on [[macOS]], [[ext2]] on [[Linux]], or [[Unix File System]] on [[Solaris (operating system)|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 [[Logical volume management|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 ([[umount|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, [[Header (computing)|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&nbsp;MB), common in [[SSD|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]] 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]] commands, such as [[Tagged Command Queuing]] and [[Native Command Queuing]] (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 command set|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&nbsp;3.0, fixes several of these issues, including command queuing, command pipes for hardware requiring them, and power management.

Specific USB&nbsp;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 {{mono|-d}} option followed by "chipset").<ref>{{cite web|url=http://sourceforge.net/apps/trac/smartmontools/ticket/25#comment:17 |title=#25 (SCSI pass through for SMART via USB on MacOSX smartmontools? 3rd party code available!) – smartmontools |website=Sourceforge.net |access-date=2014-01-21}}</ref> More recent USB storage chipsets support the [[SCSI / ATA Translation]] (SAT) as a generic protocol for interacting with ATA (and SATA) devices.<ref>{{cite web |url=http://sourceforge.net/apps/trac/smartmontools/wiki/USB |title=USB smartmontools |website=Sourceforge.net |access-date=2014-01-21 |url-status=dead |archive-url=https://web.archive.org/web/20120207000458/http://sourceforge.net/apps/trac/smartmontools/wiki/USB |archive-date=2012-02-07 }}</ref> 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.<ref>{{cite web|url=https://ata.wiki.kernel.org/index.php/ATA_Secure_Erase |title=ATA Secure Erase - ata Wiki |website=Ata.wiki.kernel.org |date=2013-07-22 |access-date=2014-01-21}}</ref>

==See also==
* [[USB#Device classes|List of USB Device Classes]]
* [[Disk encryption software]]
* [[Media Transfer Protocol]]
* [[Picture Transfer Protocol]]
* [[SCSI / ATA Translation]]

==References==
{{Reflist}}

== Further reading ==
From the USB Implementers Forum website:

* [https://usb.org/document-library/mass-storage-class-specification-overview-14 Mass Storage Class Specification Overview 1.4]
* [https://usb.org/document-library/mass-storage-bootability-specification-10 Mass Storage Bootability Specification 1.0]
* [https://usb.org/document-library/mass-storage-bulk-only-10 "Mass Storage Bulk Only 1.0" ]

== External links ==
* [https://cgit.freebsd.org/src/tree/sys/dev/usb/storage/ USB Mass Storage Device source code in FreeBSD]
* [https://www.technovelty.org/linux/what-actually-happens-when-you-plug-in-a-usb-device.html What actually happens when you plug in a USB device?]{{snd}} Linux kernel internals

{{USB}}
{{Computer-bus}}

[[Category:Computer storage buses]]
[[Category:Computer storage devices]]
[[Category:USB|Mass storage device class]]