I would be suspicious of the HDD at this point. Go to the WD website (link below) and get Data Lifeguard Diagnostic for Windows. Make sure your HDD is operating correctly. This program will test the integrity and operation of your HDD by performing some diagnostic tests.http://support.wdc.com/product/download.asp?groupid=606&lang=en
If not healthy, replace the HDD. If all checks out okay, let us know and we can troubleshoot some more.