After multiple failed attempts to get Time Machine to complete a full initial backup, I seem to have hit upon a workaround solution.
I noticed in Activity Monitor that two apps were always eating up a LOT of processor cycles during the failing backups - SPOTLIGHT was apparently trying to index the new backup disk as it was being written and I think NORTON SECURITY was also trying to virus-scan it at the same time.
- I erased the old Time Machine drive with Disk Utility and gave it a new name
- I shut off all Norton scans temporarily
- System Preferences>Spotlight>"Excluded" the new Time Machine drive from indexing and also the entire Macintosh HD (temporarily)
- Time Machine> unchecked "Back Up Automatically"
- Deleted Macintosh HD>Library>Preferences>com.apple.TimeMachine.plist and emptied the Trash
- Restarted the computer
- Quit Norton Security
- Double-checked to make sure everything was Excluded from Spotlight indexing
- Time Machine> deleted the prior Time Machine drive name and replaced it with the newly created one
- Time Machine> checked "Back Up Automatically" and started backup
- This time the full 390GB backup ran at "light speed" and completed in about two hours (via USB 3.1)
- Restored Norton to original settings
- Removed the Spotlight "Exclusion" for Macintosh HD, but retained the exclusion of the new Time Machine drive
- Restarted the computer and let it resume "business as usual".
PROBLEM SOLVED! You're welcome. :-)
