As I think about it more, something doesn't quite add up here:
On a IIgs, a 3.5 drive must only respond when pin 4 is high.
On a Mac, pin 4 will always be low (GND). So a 3.5 drive designed for the IIgs will never respond.
But we know there are some floppy drives that work on both the IIgs and the Mac. How can that be? There doesn't seem to be any way the drive could know what kind of computer it's connected to.
Edit: Maybe the drive could internally have a pull-down resistor on pin 9. On the Mac that's a no-connect, so it would always be low. But on a IIgs it would normally be driven high. Then the drive could be designed to respond if pin 4 is high, pin 9 is high, and pin 17 is low (IIgs mode), or if pin 4 is low, pin 9 is low (thanks to the pull-down), and pin 17 is low (Mac mode). That seems pretty wacky, though.
On a IIgs, a 3.5 drive must only respond when pin 4 is high.
On a Mac, pin 4 will always be low (GND). So a 3.5 drive designed for the IIgs will never respond.
But we know there are some floppy drives that work on both the IIgs and the Mac. How can that be? There doesn't seem to be any way the drive could know what kind of computer it's connected to.
Edit: Maybe the drive could internally have a pull-down resistor on pin 9. On the Mac that's a no-connect, so it would always be low. But on a IIgs it would normally be driven high. Then the drive could be designed to respond if pin 4 is high, pin 9 is high, and pin 17 is low (IIgs mode), or if pin 4 is low, pin 9 is low (thanks to the pull-down), and pin 17 is low (Mac mode). That seems pretty wacky, though.