It had nothing to do with the Superdrive. They are electrically compatible. You have a bigger problem. A 400K drive CANNOT be used on a SWIM equipped Mac, but that's only because the PWM signal is not present to step the drive motor and is instead replaced by a 5V signal which can damage the 400K drive.
The Superdrive automatically senses the PWM input on pin 20 and defaults to 400/800K operation only. It requires a steady 5V signal to operate as a 1.44MB HD drive, which only a SWIM equipped Mac provides. I have used Superdrives on my stock 128K & 512K Macs without any difficulties.
The Superdrive draws more current than the 400K and the increased load probably was the last straw that pushed your borderline 512K over the edge.