That's an excellent suggestion and now my current most probable possibility. This morning I tried booting up a fresh windows install with my Hitachi drive. The two SATAs were also connected but moved back in the boot sequence to ensure that they would be drawing some power but not interfering. After those tests, I switched my hard drive boot order back to what it was. I got out a stopwatch after the Hitachi attempts, so the numbers for that drive are only estimates. Here are my results:
Hitachi Attempt 1: 5 seconds to pane, 35 seconds to reach desktop
Hitachi 2: 1 second (if even) to pane, 7 seconds to desktop
WD 1: 1:45 of no pane, give up
WD 2: 1:00 of no pane, give up
WD 3: 22 seconds of no pane, 1:51 to reach the desktop
Wd 4: 4 seconds to reach pane, 30 seconds to reach desktop
While this isn't indisputable proof that my psu is failing, and sadly I don't have a multimeter, I'm still considering buying that psu. Here's something else I want your opinion on though: Way back four years ago when I bought this thing I wanted to overclock it. I would set everything correctly in the BIOS for a modest overclock, yet when I booted it would always run at factory speeds. I ended up updating the BIOS version on my mobo as well as a million other things, and I could never get it to listen the settings I put in the BIOS. How plausible is it that this is a result of my psu being only a 400w unit? Could my mobo be built to reset the CPU's clock if it can't draw enough power or somesuch?