Sorry, this is a tough one. Your logical process is sound, you removed everything, added things slowly, and seemed to have isolated the problem component - the video card. Kudos on that.
As to why the video card would generate a disk error I don't know, the only thing I can possibly think of, is what you thought of, power disruption to the hard drive.
Sorry, I got nothing for you. I guess you could try another PSU, or try running with just the hard drive and video card, maybe without any other peripherals power wouldn't be a problem. Good luck.