A floppy disk is magnetic storage medium for computer systems. The floppy disk is composed of a thin and flexible magnetic disk sealed in a square plastic carrier. In order to read and write data from a floppy disk, a computer system must have a floppy disk drive (FDD). A floppy disk is also referred to simply as a 'floppy.' Since the early days of personal computing, floppy disks were widely used to distribute software, transfer files and create back-up copies of data. When hard drives were still very expensive, floppy disks were also used to store the operating system of a computer.