I had an intermittent issue with a hard drive once similar to yours, turned out to be the RAM i think, i removed each ram stick one at a time rebooting between, then put them back one at a time rebooting between and it cleared it up???? one of the sticks must of messed up and would't clear itself i expect, or it was just coincedental that i was messing with the RAM when the issue solved itself... who knows :P