First, make sure that your hd82 (your 80GB drive set as Master on 2nd adaptor) hasn't been accidentally disconnected from the flat data cable or its power connector during the changing of the drives. It shouldn't be seeing this sort of problem.
You may get better speed from this drive by making it hd81, you change the hd80 (20GB) drive jumper to master with slave and the jumper on the hd81 (80GB) to slave - or both to cable select if your 80-line UDMA cable is a CS-type.
Go into your BIOS (delete, F1, F2, F10, Escape, or Control key during a bootup) and see if the 80GB drive recognises. If not, shut down, then unplug, then reconnect, the flat cable and 4 pin power feed to the drive and try again.