U disk, full name "USB flash drive", English name "USB flash drive". It is a miniature large-capacity mobile storage product, with USB interface, without physical drive, and can be connected with a computer through USB interface to realize plug and play. The name of USB flash drive comes from a new storage device produced by Netac, called USB flash drive, which is connected through USB interface. After the USB interface is connected to the computer host, you can exchange the data of the U disk with the computer. However, the equipment with similar technology produced later, because Netac has registered the patent, can no longer be called "USB flash drive", but changed its name to "USB flash drive". Later, the name U disk was widely known because it was simple and easy to remember. Until now, both are commonly used, and they are no longer distinguished. It is one of the mobile storage devices. The drive letter is the identifier of the DOS and WINDOWS systems of the disk storage device. Usually, it is identified by 26 English characters plus a colon:. Due to historical reasons, early PCs were generally equipped with two floppy disks, so the letters "a:" and "b:" were used to represent floppy disks. The sizes of early floppy disks were 8 inches, 5 inches and 3.5 inches. Hard disk devices begin with the letter c: and end with the letter z:. For UNIX and LINUX systems, there is no concept of drive letter, but the concept of directory and path is the same.