William Lindley wrote:
> * make sure the floppy is in the end of the cable after the
>   twist (due to IBM's 1981 decision to include only a 62 Watt
>   power supply which couldn't support both floppy drive motors
>   being on at once, and the clone makers who have slavishly
>   followed that brain-damaged design down to the present day)

The reason the drive cable had the twist was for inventory 
reduction.  All drives are set to DS1 (drive B) so IBM wouldn't
have to carry drives pre-selected to DS0 and DS1.  IBM didn't 
trust the technicians to move the jumper.

If you have ever done a two drive file copy, you will notice
both motors run continuously, and the red light on the front of 
the drives each flicker on/off in turn.  Both motors stay on
for 4 seconds after the light goes out, so during this operation,
both drive motors are running.  This was done on purpose in DOS 
assuming the disk may be read again quickly and this saves the 
startup time as the drive gets up to speed.

I wrote a program called diskcomp that will perform a sector by
sector comparison of two floppies.  It definitely keeps both
motors on simultaneously.

George