The reason you can't use an original 400K disk with a SWIM is that the 400K drive depends on the system clock signal (PWM) to adjust speed on pin 20. Since the 800K disk nor the Superdrive depend on this signal, adjusting their own speed internally, the SWIM uses a completely different signal on pin 20 which has nothing to do with disk speed (in particular it is +5V). Therefore, the 400K drive simply will not work on a SWIM Mac and could possibly be damaged.