Info přímo od Microsoftu z jejich knowlidge base, http://support.microsoft.com/kb/140418

Konkrétně:

Even on a non-bootable floppy disk, there is executable code in this area. The code necessary to print the familiar message, "Non-system disk or disk error" is found on most standard, MS-DOS formatted floppy disks that were not formatted with the "system" option.