I know the diode solution works. I've used it before. In the system I built in the late 80's early 90's, I used the diode method for a small motor. In the same thing, I used an LM317T as the motor power supply and I rigged it to put the motor into shutdown (1.2V output) via an SCR if the computer control failed. Power cycling was necessary to reset the protection. The motor was a small 6V plastic geared motor.