Recently I have had many problems with my hard drives, both internal and external. I had two external Western Digital drives fail. I tried Diskwarrior but it failed to fix any of my drives. I recently updated TechTool Pro and decided to use it to examine my drives. It found directory problems in both my primary drive and my external 1TB WD drive. The program repaired both my main drive and my external drive. I am now able to use my external drive (which I had gotten an RMA to return it) and my internal drive now allows me to operate normally (I previously was unable to update Microsoft Office 2008 because of the directory problems,; now I can update successfully). Thank you for your great product! Doug Unruh