It sounds that the hard drive is going bad.
Do not run any more diagnostic tests on it, and don't used it at all until You have taken a backup of the important data from it.
A hard drive that's start going bad can be easily worse from every read from it and make so data on it will be corrupt and destroyed and then you will have serious problem to restore this.
Here is the instruction and tip's for Data Lifeguard Diagnostics tool:
http://support.wdc.com/KnowledgeBase/answer.aspx?ID=940
Consider to replace the HDD.
Best regards from Sweden