The volume serial number was added to the standard format for IBM PC-compatible disks in 1987, when
Microsoft and IBM were co-developing OS/2.  They wanted the system to operate like the Macintosh, which
automatically recognised which diskette (or removable disk cartridge) had been inserted in a drive.  Up to that
point, the only identifying information on an IBM-compatible disk was its volume label.  If the user declined to
assign a name to the disk, or gave more than one disk the same name, there was no way to tell them apart.
 
    With this in mind, the two companies decided to change the disk formats for both MS-DOS and OS/2 to
include a four byte volume serial.  When a disk was formatted, it would be stamped with this number, which in
certain operating systems (see note below), was constructed from the exact date and time the format
operation was performed.  These values allowed FAT file system drivers to detect that the wrong disk had
been inserted in a removable drive.  The odds of two disks getting the same number were virtually nil on the
same machine, and were still small even if users exchanged disks with one another.
 
    When a diskette was reproduced by the operating system's disk copying program (DISKCOPY), the program
would make a faithful copy of every byte on the entire disk except for the volume serial number, which would
be changed to something different than the one on the original diskette.