Same experience drop my hardisk several time. Try the following:
1. Don't write anything or reformat. <IMPORTANT>
2. Try install Hard Disk Sentinel and see what status display i.e. health level.
3. Normally can still read data from drive.
4. Alway prompt bad sector or fail operation when you try to format it. Seem the read/write head is out of position. Cannot use again and require repair.
I replace 3 hardisks with agent under this condition.