You said you confirmed the drive works in another Mac. The fact the stepper motor doesn't work, may hint at one problem. Check the ROMS in the other Mac you tested it in. If they are Mac Plus ROMs, or 342-022x-B then your 400K drive may require the REV B ROMs or better. Apple revised the original 64K ROM once for changes in a later model stepper motor. Im not sure exactly how this problem manifests as the drive model numbers offer little help in determining which you have. However, considering everything else you've tried, it's worth a check.
http://support.apple.com/kb/TA34190?viewlocale=en_US