USB interfacing is hard, but SD cards are a lot
simpler. So use a card
reader thing to transfer the files to an SD card and design an
interface for that to ISA bus.
Thinking of that, I had actually wondered whether one could transfer the
appropriate image files from a modern PC (internet access) to a classic
PC (disk drives) using a SCSI hard drive. Not if the modern PC is a
laptop, I suppose.
Tony, I don't recall what became of the AT with 486 replacement
processor that used to be your main home machine. ISTR it had loads of
different floppy drives hung off it. Is it still functional / repairable?
Also, a passive ISA backplanes used to be fairly common. Mine has
(had?) a 486 [1], but I think there were pentium processor cards
available. I have no idea what your spares box might contribute in that
line, though.
Philip.
[1] I found the backplane in a recent clearout, but I can't remember
what cards it had in it.