How does HD Sentinel identify the drive?http://www.hdsentinel.com/
Could we see the partition table and boot sector with Microsoft's Sector Inspector? http://www.users.on.net/
If you extract the above archive to the one folder and execute the SIrun.bat file, then this will generate a report file named SIout.txt.
Alternatively, can you see sector 0 with a disc editor?
HxD - Freeware Hex Editor and Disk Editor:
DMDE (DM Disk Editor and Data Recovery): http://softdm.com/download.html