Interesting. I see several other differences in the floppy connector pin assignments between the IIgs and Mac. How are these ever able to use the same external drive?

4: IIgs is 3.5/5.25 Drive Select, Mac is GND

9: IIgs is Drive 2 Select, Mac is N.C.

10: IIgs is Write Protect, Mac is PWM

16: IIgs is Head Select, Mac is register select

17: IIgs is Drive 1 Select, Mac is external drive enable (is drive 1 external?)

Pin 10 is especially troubling, because it's an output on the Mac, and an input on the IIgs, and I don't see any way to reconcile those.

The others could work if Apple chose the right defaults. If GND = 3.5 inch drive, the external drive is drive 1, and head select can be re-mapped to register select in software.

Given that there are some real-world Apple floppy drives that will work on a Mac and a IIgs, this has to be possible somehow.

Here's some more info on Apple IIgs floppy cables that may be relevant: http://apple2.info/wiki/index.php?title=Pinouts#3.5_in._and_5.25_in._Drive_cable_pinouts