Summary: Having problems reading some old 5.25" floppies. Looking for advice on what else I can try before declaring the disks un-retrievable and giving-up.
I recently found a box of old 5.25" DSDD floppy disks dating for the about 1988-89. They are MS-DOS FAT format. They seemed in good physical condition and I don't think they'd been stored near strong magnetic fields or in extreme temperatures, so I decided to have a go a reading them. I acquired an old Compaq Deskpro 5100 XL (which as a 5.25 capable floppy controller and BIOS) that was about to be dumped and bought a reconditioned Canon MD5201 360kb drive and a new floppy cable off ebay.
I fitted the 5.25" drive and connected it and the existing 3.5" using the new cable and replaced the bios battery. The bios recognizes the 5.25" drive as drive B. The PC only has 16mb of memory so I installed NT4 SP6 on the hard drive.
The 5.25" drive spins-up when I insert a disk, and if I try to view the contents of the disk using NT file explorer then the drive access light illuminates. After about a minute I get an error dialog saying "B:\ is not accessible. The device is not ready." This happens for each disk. I decided to sacrifice one of the disks by formatting it but get the same error. If I boot the machine into dos then I get "Not ready reading drive B" when I try to read any of the disks.
I think the most likely explanation is that the disks just aren't readable after all this time. But I find it suspicious that (a) all the drives have completely failed in apparently the same way, and (b) a pack of 3.5" disks that were stored with them are perfectly readable.
I've tried using a different (old) floppy cable and a 1.22MB Teac floppy drive instead, with the same results.
Can anyone suggest anything else I should try (software/hardware) before giving up?
Edit 1: Clarified that they are MS-DOS disks
Edit 2: I had to leave this for a while, but I have now followed-up all the suggestions made by those who helpfully left answers and/or comments. After trying two different drives with every combination of cables, socket positions, and jumper settings, I've come to the conclusion that the floppies just aren't recoverable. Unfortunately this was always the most likely outcome. Thanks to all those who provided advice when there wasn't likely to be a simple answer that I could accept. I have up-voted the answers that were useful.