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The USB mass storage device class or USB MSC or UMS is a set of computing communications protocols defined by the USB Implementers Forum that run on the Universal Serial Bus. The standard provides an interface to a variety of storage devices.  Some of the devices which are connected to computers via this standard are:
Devices which support this standard are referred to as MSC (Mass Storage Class) devices. While MSC is the official abbreviation, UMS (Universal Mass Storage) has become common in on-line jargon.
[edit] Operating system supportMost current mainstream operating systems include support for USB mass storage devices, although support on older systems is available through patches. [edit] Microsoft WindowsThere is no native (supplied by Microsoft) support for USB in Windows versions prior to Windows 95 and Windows NT 4.0, or MS-DOS. Windows 95 OSR2.1, an update to the operating system, featured very limited support for USB. During that time, no generic USB mass storage driver was produced by Microsoft, even for Windows 98. This meant that a device-specific driver was needed for each type of USB storage device. Windows 2000 has support via a generic driver for mass storage devices, however a device-specific driver was required for rare devices not conforming the USB MSC specfication[citation needed]). Windows Me and all later Windows versions also include support. Today, third party generic drivers which support USB flash drives are available freely even for Windows 98 SE and Windows NT 4.0. Windows Mobile supports accessing most USB mass storage devices formatted with FAT on devices with USB Host. However, portable devices typically cannot provide enough power for disk enclosures containing hard drives (a 2.5" hard drive typically requires the maximum 2.5 W provided by the USB specification) without a self-powered USB hub. On the other way around, Windows Mobile devices can not show their file systems as a mass storage device unless the device implementer explicitly decides to add such functionality. However, third party applications exist to add MSC emulation to most WM devices (commercial "Softick CardExport" and free "WM5torage"). Generally only memory cards can be exported, and not internal storage memory, due to the complications outlined below. FreeDOS supports USB mass storage as ASPI devices. [edit] Malware and inherent vulnerabilitySince Windows's AutoRun feature works indiscriminately on any removable media, USB storage devices became the infection entryway for computer viruses. As the FAT file system, most used on USB storage for its simplicity and wide compatibility, has no access control features, a user has no convenient way to protect his USB drives from infection after inserting into untrusted computer unless the device has a hardware read-only switch. [edit] Mac OSApple Computer's Mac OS 8.5.1 supports USB mass storage through an optional driver. Mac OS 9 and Mac OS X support USB mass storage natively. [edit] Unix-likeThe Linux kernel has supported USB mass storage devices via a generic driver since kernel version 2.4 (year 2001), and a backport to kernel 2.2 has also been made. Solaris also supports devices since the 2.8 release and up (year 1998), NetBSD since the 1.5 release and up (year 2000), FreeBSD since the 4.0 release and up (year 2000), and OpenBSD since the 2.7 release and up (year 2000). [edit] DOSDOS has no generic support for USB but there are external drivers available which support USB mass storage devices. These are Duse, USBASPI and DOSUSB. [edit] AmigaOSAmigaOS supports UMASS storage devices through Poseidon, a third-party USB stack which has become a de facto standard. It supports various USB device types through a modular system of Hardware Independent Device Driver (HIDD) classes. Poseidon as used in AmigaOS Classic up to version 4.0, and in the MorphOS operating system. The supposedly final AmigaOS version of the Poseidon stack is released OEM licensed with the Deneb USB card in May 2008. AmigaOS 4.0 has its own USB stack called Sirion, though it can still use Poseidon. A new USB stack called ANAIIS (Another Native Amiga IO Interface Stack) is available for all Amiga platforms with Highway or Subway hardware, but does not yet support UMASS. FAT16 and FAT32 filesystems are supported by the FAT95 filesystem. In September 2009 Poseidon was released as Open Source and was finally also ported to AROS, an Open Source AmigaOS inspired system. [edit] Game consolesThe Xbox 360 and PlayStation 3 support most mass storage devices. [edit] Graphing CalculatorsIndependent developers have released drivers for the TI-84 Plus and TI-84 Plus Silver Edition in order to access USB mass storage devices. usb8x contains the driver for this access, while msd8x handles the user interface. [edit] Device access The USB mass storage specification does not require any particular file system to be used on conforming devices. Instead, it provides a simple interface to read and write sectors of data—much like the low-level interface used to access any hard drive—using the "SCSI transparent command set." Operating systems may treat the USB drive like a hard drive, and can format it with any file system they like. Because of its ubiquity and relative simplicity, the most common file system on embedded devices such as USB flash drives, cameras, or digital audio players is Microsoft's FAT or FAT32 file system with (optional) support for long names. Large USB-based hard disks may come formatted with NTFS, which is much less supported outside Microsoft Windows. However, a keydrive or any other device may be formatted using another filesystem (for example HFS Plus on an Apple Macintosh, or Ext2 under Linux, or Unix File System under Solaris or BSD). Of course, this choice may limit other operating systems' ability to access the contents of the device. In cameras, MP3 players, and similar gadgets which must access the file system independently from an external host, the FAT filesystem is typically preferred by device manufacturers. [edit] Complications of the mass-storage device class[edit] Hard drive-based devicesMany modern hard drives support additional advanced commands, such as Native Command Queuing, which may increase performance, and S.M.A.R.T., which allows a computer to measure various indicators of drive reliability. These exist as extensions to the basic low-level command sets used by hard drives, such as SCSI, SATA, or PATA. These features do not work when hard drives are encapsulated in a disk enclosure supporting the USB mass storage interface. USB mass storage provides a generic interface which only provides basic read/write commands, as outlined above. This works perfectly well for basic data transfer using hard drive-based devices, however it means that there is no simple way to send advanced, device-specific commands to USB mass storage devices (although devices may create their own communication protocols over the USB-standard "control interface"). The new eSATA standard for external drives promises to address this issue, since it extends the internal SATA bus to external disks without any intermediate translation layer. [edit] See also[edit] External links
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