Hi Parcel32.
That fine, now you have a backup on the separate drive and over 400 GB free on the main backup drive we can start again with no difficulty but would you mind waiting a while to see if the Norton team can use your case to find out what appears to have caused this growth of the backup file? I know that it was no where near as bad as you first thought but it is still pretty big!
If you do not want to wait I will still gladly advise you on how to clear up and start again.
Are you content to wait a while?