(1) An external hard drive that plugs into the computer via USB. See
portable hard drive.
(2) A reference to booting the computer from a USB drive. In the computer's startup routines, users can specify the bootable device sequence. For example, if a USB drive is first and the main storage drive is second, the computer will boot from the USB drive if one is plugged in (see
BIOS). See
first boot sequence.