USB Mass Storage

The USB mass storage device class (also known as USB MSC or UMS) is a set of computing 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 set interfaces with a number of storage devices.

Devices connected to computers via this standard include:

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.

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

Microsoft Windows has supported MSC since Windows 2000 (Windows NT5). 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 FAT on devices with USB Host. However, portable devices typically cannot provide enough power for hard-drive disk enclosures (a 2.5-inch (64 mm) 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-systems issues; see device access, below.

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