Sounds similar to a problem I had a while back. Turned out to be an otherwise OK hard drive that sometimes didn't want to spin up to speed without pulling way to much juice (especially when it was still warm and stopped!), enough that the MB would fail to boot and hold the ATX PS-on line steady. Machine would hit the beep during boot, then power off leaving the power light on, a kind of half-way emergency shutdown.
It's actually this kind of HDD issue that has me working HDD cooling into my setup. On the one hand, it's not strictly needed, on the other hand I've got fast spinning drives that will hopefully spin continuously for several years at a time - so a few degrees operating temperature will make a significant MTBF difference.
Funny thing was the same drive would do ok with a different MB 99% of the time. Apparently that MB was better able to maintain its voltages than the other. Might have been cheap Chinese caps on the board that had the most trouble with it.
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