I just removed both hardrives AND the IDE cable completely from the PC this time.
I powered it on, set both hardrives to "none" and shut it down, then I re-inserted both hardrives, with correct jumper configuration, master and slave and everything PERFECTLY WELL, the IDE cable is ALL the way in in the motherboard, changed the hardrives from None to Auto, restarted and....
STILL NOTHING !!!!!!!!!!
What's left to do ? I gotta try with another IDE cable at this point...
Currently assembled vintage computers I own: 11
Most important ones:
A "modded" Olivetti M4 434 S (currently broken).
An Epson El Plus 386DX running MS-DOS 6.22 (currently broken).
Celeron Coppermine 1.10GHz on an M754LMRTP motherboard