What is your evidence that an (unmodified) pc floppy drive can run at variable spindle speeds? The interface defines simply a "motor on" signal and it's not a PWM line. Devices like the Catweasel and the old Central Point boards can read Mac GCR disks because their data seperator circuitry supports multiple bitrates with a *fixed* spindle speed. (If you want to do the trick Apple did with cramming more data into the "longer" tracks you can do it equally well by varying the speed at which you clock the data. Better, in fact, because you don't have to wait for the disk to speed up or slow down after stepping the head. Old Commodore floppy drives like the 8050 use this technique. )