Hello barneycanes,
Sorry for the trouble. Please let me know the version of Norton Security Suite you use, you can find it from Support> About in Norton program. Now, go to Tasks> Check Security History and select Backup from the drop down. Click on more details on the right pane for the last backup you ran. This problem may happen if some files are not properly backed up, so please verify if the details in security history shows any such problems.
As a test, you can completely remove the current backup set and create a new backup set with just one file(temporary purpose), then run the back to see if that completes successfully. Let us know the results.
Thanks!
Harry