So I tried plugging the problem HD into every SATA connector from my PS, even the ones going to my other HDs. Also tried the 4-prong to SATA adapters. Tried each with and without data cable plugged in.
In every case, I toggle the PS, plug in HD, and punch power button, and the MB lights...
I recently bought a Seagate Barracuda 7200.10 750GB, and it worked as expected for a month or so. Then for a week it was stored unprotected and unplugged near the main tower. Now when I plug in the SATA power connector, the entire machine immediately loses power. Pressing the power button...