Symptoms of a weak power supply?

Are you running overclocked? If so, disable your OC. If not, try bumping up the voltage to your CPU anyway. It might just be a chip that was slightly out of margin, but not caught, and when you added your hard drives, it may have pushed it over the edge.

Reason i mention this, is because after running my i7 at 4.0 for two years, i had to turn the voltage up to sustain it, and it's to the point now where it won't run at stock clock on stock voltage, and i got much the same issues as you.

Also, check your IOH/QPI temp if possible.

I've only got personal experience of two situations
where an under-power psu was at fault:

1) Me, about 2 years ago. Cheap case that came with an even cheaper
psu. When I upgraded to a 1900+ athlon, stuck in a dvd and 2nd hd
(I know, Iknow), I suffered random lockups. Sought help here, took
the advice I was given, and dropped in a decent 350w PSU. Problem
solved.

2) A friend of mine rjust now. New build wouldn't boot properly from
the power switch - he'd switch it on, it wouldn't even post. Then if
he hit the reset button it would boot normally. I surmised that the
(dirt-cheap) psu couldn't supply the juice to power up the board, ram
graphics, ram etc *and* spin up all the drives for post. I passed on
the 350w psu, since it was now a spare, and his machine boots fine.

Best, Aaron