Norton error message on backup attempts

I plug-in my backup device that has previously worked fine to backup my computer hard drive.  I click on Norton 360 backup and tell it to get busy.  Norton 360 responds:  "There is a backup problem.  Make sure the destination drive or device is connected and then try again."  After dozens of attempts, shut-downs, restarts, I get the same response from Norton 360.

How do I fix the error?  There is no link to a solution in the error message.  Mechanical Nathan tech support has been no help.