HD scan in Mountain Lion I suspect a faulty drive. Anything that could perform a hardware scan (bad sectors?) built into Mountain Lion or available for free off the 'net?
You can try Disk Utility. Click Verify Disk to see if the disk is faulty.

My last HD failed after a year of use. The past month has made the computer really slow. Most HD manufacturer gives out a replacement if it fails within 3 years though.