Yeah, I didn't seriously think the mismatched clock rate would be the problem, the thought just crossed my mind and I tossed it out there. There's a table in the IWM's datasheet that suggests the mismatching clocks should be well within the IWM's tolerances, so, yeah, never mind. (It is *slightly* interesting in that the clocking suggests there would be a reliably detectable difference in bit density between disks written on an Apple IIgs and a Mac Plus, but again, not enough to worry about.)

As for the origin of the IWM the "Twiggy" drive project supposedly goes back to around 1979, and the IWM was specifically designed (IE, with its "fast" mode) to drive the Twiggy... which does make it interesting/ironic that the part wasn't actually ready in time for the original Lisas.