I would examine sector 0 with a disc editor. That's where the partition table lives.
DMDE (DM Disk Editor and Data Recovery): http://softdm.com/download.html
I would also run a test on the drive using either of the following utilities.
HD Sentinel (DOS / Windows / Linux): http://www.hdsentinel.com/
HDDScan for Windows: http://hddscan.com/