(v.) (1) To prepare a storage medium, usually a disk, for reading and writing. When you format a disk, the operating system erases all bookkeeping information on the disk, tests the disk to make sure all sectors are reliable, marks bad sectors (that is, those that are scratched), and creates internal address tables that it later uses to locate information. You must format a disk before you can use it.
Note that reformatting a disk does not erase the data on the disk, only the address tables. Do not panic, therefore, if you accidentally reformat a disk that has useful data. A computer specialist should be able to recover most, if not all, of the information on the disk. You can also buy programs that enable you to recover a disk yourself.
The previous discussion, however, applies only to high-level formats, the type of formats that most users execute. In addition, hard disks have a low-level format, which sets certain properties of the disk such as the interleave factor. The low-level format also determines what type of disk controller can access the disk (e.g., RLL or MFM).
Almost all hard disks that you purchase have already had a low-level format. It is not necessary, therefore, to perform a low-level format yourself unless you want to change the interleave factor or make the disk accessible by a different type of disk controller. Performing a low-level format erases all data on the disk.