Yep....I had the same problem about 3 months ago on a second nertwork computer in my studio. It was the IDE Cable. You might want to replace it before you get crazy looking for viruses etc.
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EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.