Root cause analysis of the spark determined that I managed to somehow overcome the keying on the MOLEX->SATA power connector -- ultimate I was putting +12VDC on the pin that was expecting +5, I was putting +5 VDC on the pin that was expecting +12 and I swapped the grounds around too (the ground-swapping was the least of this drive's worries). If what I'm saying doesn't make sense, look here. In short, "I made the yellow go to red and the red go to yellow," because I am a complete idiot. Fucking good thing I took a snapshot with Acronis of the drive before I started all this effort or Curmudgeon would be well and truly boned by my ineptitude.
I am not a happy camper.
I'm going to bed now. If I'm very lucky I won't accidentally cause the bed to explode, or something.