The FORMAT command rebuilds an empty file system on a floppy disk, removable-media disk, or a hard disk. In the process, the contents of the disk are overwritten, or the index of the files is erased ('quick' format).

FORMAT works in very different ways, depending on whether it is used on a hard or floppy disk. When Format is used on a hard drive, it creates a master boot record, two file allocation tables, and a root directory (also referred to as the root folder). The rest of the drive is checked for disk surface errors—any defective areas are marked as bad to prevent their use by the operating system. Format appears to “destroy” the previous contents of a hard disk, but if you use Format on a hard disk by mistake, third-party data recovery programs can be used to retrieve data from the drive. This is possible because most of the disk surface is not changed by Format.

If a floppy disk, USB flash memory drive, or removable-media disk is prepared with Format and the unconditional /U option is used from the command line, or the Windows Explorer Full Format option is used, sector markings (a sector equals 512 bytes) are created across the surface of the floppy disk before other disk structures are created, destroying any previous data on the disk. If the Quick Format or Safe Format option is used, the contents of the disk are marked for deletion but can be retrieved with third-party data recovery software.

Note: The hard disk format process performed by the Format command (which creates the file system) is sometimes referred to as a high-level format to distinguish it from the low-level format used by hard drive manufacturers to set up magnetic structures on the hard drive. When floppy disks are formatted with the Full or Unconditional options, Format performs both a low-level and high-level format on the floppy disk surface. Completely erasing a hard drive with a magnetic bulk-eraser makes the drive permanently unusable as the low-level formatting can't be put back on a hard drive with FORMAT.