>  Now both 720K and 1.2M drives use 80 tracks, and the motor speed is
>fixed.  What happens is that in 1.2M mode, the data rate is doubled
>*and* the write current is doubled as well, also the receive amplifier
>compensation is changed.  Same speed, double rate, density is doubled.

I believe the 1.2M floppy was designed as a 5.25 inch version of the 8 inch
floppy, i.e. same number of tracks and same data rate. The data bits per track
for a 360KB 5.25 inch floppy is not half the amount of an 8 inch floppy. Hence
the spindle speed is changed in 1.2M mode to match the data rate of an 8 inch,
which means less bits per inch in this mode.