Norton Ghost 15 - backup failure

I have been using NG15 for over a year successfully w/o issue.  I backup my entire computer daily to a 3Tb Western Digital hard drive.  Been using E:MyBook.  Suddenly NG15 successfully saves but WD hard drive is 'unavailable'.  Hard drive is healthy.

 

Reassigned via Windows XP to G drive.  Backups still fail to be saved. 

 

This must have a simple solution.  Anyone have a suggestion?