Time Machine stuck Preparing Backup...

Time Machine was working well, then began getting stuck on Preparing Backup...

Things tried:

— Letting ”Prepare Backup…” run overnight

— Turned Back Up Automatically off/on

— Added back up drive to Privacy in Spotlight

— Removed the .inProgress File

— Removed/Selected Disk

— Restarted

— Ran First Aid, all good

— Checked to see if Spotlight was indexing, it wasn’t

— Checked Activity Monitor to see if mdworker has in play, it wasn’t

I got it working again once by wiping the drive and starting again (27 hour initial back up). Thought all was good until it got stuck immediately on subsequent back up.

‘Subsequent backups’ seem to be the issue, maybe?

Anything else I can try? 

Thanks,

OS High Sierra 10.13.6

No antivirus software 

No software updates

Nothing new/different was installed

New external USB drive healthy/plenty of room