Elfen said:

Warning: This is also why you can not use a Apple IIGS 3.5 floppy on a Mac - both the wires and the drive board inside are different. Using it will blow out your IWM/SIWM Chip and/or the drive. You can recycle the mechanicals from a Apple IIGS 3.5 to fix a Mac 800K floppy (and visa versa) but not the electronics.

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What are you talking about? The A9M0106 drive specifically intended for the IIgs is a "universal" drive and works with both Macs and Apple II's equipped with suitable controller cards, and that includes the controller built into the IIgs. I have one of these drives and use it with my 512k; it has the advantage over the Mac-styled M0131 800k external disk (sans eject button) in that it works fine as a 400k drive if you don't have the HD20 INIT installed on your boot floppy. (And therefore is itself bootable, unlike the M0131.)

Are you sure you're not thinking of the "Unidisk 3.5" drive, A2M2053, that was mostly sold for use with the original IIc? *That* has a magical control board inside the case that, yes, makes it incompatible with a Mac.