Volume serial number
A volume serial number is a serial number assigned to a disk volume or tape volume. It originated in 1950s in mainframe computer operating systems. In OS/360 line it is human-configurable, has a maximum length of six characters, is in uppercase, must start with a letter, and identifies a volume to the system in unique manner. For example, "SYSRES" is often used for a system residence volume.
In FAT and NTFS file systems, a volume serial number is a feature used to determine if a disk is present in a drive or not, and to detect if it was exchanged with another one. This identification system was created by Microsoft and IBM during their development of OS/2. It was introduced in MS-DOS 4.01 in 1988.
The volume serial number is a 32-bit number determined by the date and time on the real-time clock on the current computer at the time of a disk's formatting. Previously, determination by the OS of whether a disk was swapped was done by reading the drive's volume label (much similar in concept to OS/360). However, even at that time the volume label was not required to be unique and was optional. Therefore, many users had not given disks any meaningful name and the old method failed.