The problem I was having, that my machine hung in the BIOS when I added a UDMA100 disk is now solved.
Yes, I fitted a UDMA100 disk last night and it seemed to work fine. Except, perhaps, that cfdisk thought it only had 8 gig (as opposed to 80). the Bios seems to think it's 200Gig too. Weird. fdisk knew the score, though.
Interestingly, the second HD on that IDE channel was showing signs of temporarily hanging, but then that was *that other OS*, which I did an emergency install of until I get my Debian disks back.