This is not really any help, as the MOTOR ON signal is handled by a different part of the controller and is applicable to ALL drives connected.
I didn't realize there was only one Motor ON signal. Thanks for the clarification!
... a 'tap' idc connector....
Where EXACTLY are you tapping this cable? Im sure you're aware that the connectors on the ORIGINAL drive cable have ALL BUT THE desired drive connector REMOVED... and the drives are normally active for ANY DRIVE so that placement on the cable determines 0 or 1...
The "tap" IDC connector is a Male Pin smash on for flat cable. I 'smashed it on' close to the motherboard.
edit: Did some more looking... THERE ARE NO LINES connected for the :2 and :3 drives...
U32 (74ls174) is responsible for the :0 and :1 signal on pins 2 and 5...
pins 7 and 10 WOULD (I believe) be correct for :2 and :3 but are NOT connected to the INTERNAL FDC header and are NOT routed to the external bus...
I found directions somewhere (can't locate it now) that extends the unused pins on the bus driver to make the connector look like the connector on a Model 4 desk type. There was a clarification about the schematic having the IC marked with the wrong number on the schematic. (See your last edit
LDOS may be claiming it is illegal bcause it expects only 2 drives to be available...
I fear you may be SOL on this one! :
(Im getting good at this... !)
Another thought...
IF you can find room on the U20 (7416 hex inverter)... possibly pins 7-8 and 9-10 ? you could feed the U32 lines thru them and to the FDC header... but I cannot confirm that pins 7 and 10 ARE :2 and :3...
Ya know... Im getting tired of editing this!
Please provide some details of the 'hardware mod' you mentioned in your FIRST POST... it may not be quite right, or not working as expected...