Did you put those disks into a windows 2000/xp or
newer pc? Possibly as far
back as win 98/95 (I don't remember when they started) the boot sector gets
overwritten if you put a non write protected disk in the drive in windows.
I knew I had good reasons for disliking Microsoft operating systems. That
is plain ridiculous. No OS should overwrite a boot sector, or for that
matter just about any other part of a disk without the user explicitily
telling it to. Period.
-tony