Magnetic disks come in different sizes. The size of a disk is usually referred to by its diameter. Typical disk sizes include those with 14-inch, 9-inch, 8-inch, 5(1/4-inch), 3(1/4-inch), diameter. Obviously, for proper match of dimensions, different size disks require different disk drives. Even for disk of same size, all disk drives are not the same because disk drives of different computers often have their own away of defining tracks, sectors, and sector size (number of bytes/sector) to match their own way of organizing data. This implies that computer manufacturers should also manufacture the disks used in their computer systems. This is a severe limitation because it prevents use of disks manufactured by third party vendors into one's own computer system. To overcome this problem, earlier computer systems provided low-level disk formatting utility. It enabled a user to prepare (format) a new disk before using it with the computer system. For this, the user had to insert the raw (unformatted) disk in the disk drive of the computer system and initiate disk-formatting command. The read/write head of the disk drive then laid down a magnetic pattern on the disk's surface, making it compatible for use with it.
Modern disk drives do not require low-level formatting because modern disk controllers present consistent view of disk geometry to the computer while hiding the internal hardware arrangement. They accept and can work directly with disks that are factory formatted by their manufacturers. In fact, running a generic low-level format on these disks can cause irreparable damage to drive hardware. For this reason, modern computer system do not provide low-level disk formatting utility.
To store files and data on a disk, the Operating System (OS) creates a file-system on the disk. It maintains a table with the sector and track locations of data on the disk. This table, known as File Allocation Table (FAT), enables the computer to locate data easily. The Operating System's disk formatting command takes care of scanning and marking bad sectors, creating the FAT, and setting aside sufficient disk space for it.