Cocoa error 513 - cannot change backup hard drive?

Hi, I have been successfully using a second internal hard-drive to backup via Timemachine. I changed the back up to an external hard-drive to get another backup, which worked. However, when I tried to change back to the internal hard-drive, I get the error message "operation could not be completed (Cocoa error 513)." I have check I have the latest software updates, repaired the internal back-up drive (via disk utility - no problems), deleted the timemachine.plist file, but still the error persists. Any advice most welcome.