I've got a giant stack of 5.25" floppy disks and four 5.25" drives, including a drive that was completely unused/new-in-box.

All of the floppies fail to read with a "General failure" error, and all emerge damaged in precisely the same way from ANY of my drives.

I would assume a single damaged drive could do this to a floppy disk, but it's happened with FOUR different drives. My assumption is that the floppies are just too old/damaged to be readable, that's probably the coating itself rubbing off.

I've tried cleaning a few (gently), and there's a light-brown residue on the cleaning cloth afterwards, so that's probably NOT a good sign.

I've also cleaned the floppy drive heads with both a cleaning diskette and a q-tip with alcohol, no change.

They were stored in (presumably) very hot and damp environments over the past 30 years, so it's hardly surprising. The 3.5" disks stored along with these 5.25 were almost all unreadable as well.