My 5125 Portable had been working great since I recapped it back in 2008. Then I started to get "This (floppy) disk is unreadable. Eject | Initialize" error messages. Now it happens regardless of whether I have a disk in the drive or not. And if I do insert a floppy and attempt to initialize it, I get a "Disk locked" message even if it's not.

I made a mess of that original floppy drive, but I had an identical unit on hand. But it gives me exactly the same error messages! Tried it in a different computer with different cable, and the problem appears there too.

So far I've cleaned & lubed the drive, checked ESR of the aluminum and tantalum electrolytic capacitors (they seem OK), verified that the various switches are working as expected when a disk is inserted. All stepper motors spin, though the one which drives the heads never budges unless I manually move the head assembly off it's parked position.

I'm baffled: Has anyone seen this sort of behavior before?