BOAC,
I had a situation some years ago on a PC with 2 hard disks, when one of the 2 disk (the Win2K) disk sometimes didn't spin-up in time for the POST to recognise it, so the system booted off the 2nd disk, which was still bootable with Win98.
Neither disk was dodgy, but I eventually concluded that it was a PSU problem, and after changing it, the problem never re-occurred.
SD