A USB flash drive is essentially NAND-type flash memory integrated with a USB 1.1 or 2.0 interface. It is a small, lightweight, removable and rewritable data storage device. The price per megabyte is very high for the drives with a small amount of storage space, decreases rapidly as you move up the commonly availible sizes (currently up to around 2 gigabytes) and then increases rapidly again for drives with a large amount of storage space. Drives of up to 64 gigabytes (and possiblly higher) exist but are difficult to obtain and expensive.

USB flash drives are faster and more reliable than floppy disks which were previously used for relatively quick portable storage. These types of drives use the USB mass storage standard, supported natively by operating systems such as Linux (since the 2.4 kernel series[1]), Mac OS X, Mac OS Version 9, Windows XP, Windows 2000 and Windows Me.

Earlier versions of Microsoft Windows (from Windows 95 OSR2.1 through Windows 98 SE)[2] do not natively support USB mass storage devices, though they do support USB; in order to use a USB flash drive with these versions of Windows, a driver from the manufacturer (generally available for Windows 98 but not for Windows 95) must be installed. Also, Mac OS 8.5.1 and up to (but not including) Mac OS 9 supports USB mass storage devices through use of an optional driver.

Some recent USB flash drives act as two drives - as a removable disk device (the actual drive itself), and as a USB floppy drive (again, as the actual drive itself, but as another drive in Windows). This is likely intended to make it easier to use them as a bootable device.

USB flash drives are also known as "pen drives", "chip sticks" (though very uncommonly), "thumb drives", "flash drives", "USB keys", and a wide variety of other names. They are also sometimes called memory sticks, which can lead to confusion because memory stick is a Sony trademark for their proprietary memory card system.

A flash drive consists of a small printed circuit board encased in a robust plastic[3] or metal casing[4], making the drive sturdy enough to be carried about in a pocket, as a keyfob, or on a lanyard. Only the USB connector protrudes from this protection, and is usually covered by a removable cap. Most flash drives feature the standard type-A USB connection allowing them to be connected directly to a port on a personal computer. Some small drives have been made with a thin plug designed to mate with a standard USB port[5] but these are very rare.

Most flash drives are active only when powered by a USB computer connection, and require no other external power source or battery power source; key drives are run off the limited supply afforded by the USB connection (5 volts and up to 500 mA). To access the data stored in a flash drive, the flash drive must be connected to a computer, either by direct connection to the computer's USB port or via a USB hub. Some need most of the power a standard USB port can supply and therefore cannot be used with a bus powered hub.

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http://en.wikipedia.org/wiki/USB_flash_drive

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