In my latest battle to get this old stuff working, I have come across yet another mystery. Hardware -

486 mobo, with 1 VLB slot
VLB IO controller in VLB slot
old HDD attached to VLB IO controller

Inside BIOS, HDD is recognised OK. On boot up, POST info table displays OK. But at the point when the HDD should boot up the OS, the HDD actually seems to turn itself off! You can hear it spin down to silence. Strange. 😕