Ti Stopped Backing Up To Spare Internal Drive

Windows 7 64 bit.....

I have been backing up to a 1TB internal spare drive for a couple of years now and just recently TI won't back up to that drive..It just runs and does nothing..

The drive is good because I can copy other things to it with no problem..

I can back up to another internal drive with out any problems but the 1TB is my drive of choice..

I ran checks scans on the drive and it is fine..

Any suggestions on what's wrong and how to fix it will be appreciated.
Thanks