by Ed Burt.
Thanks for that - lots to be working through from a diagnosis/resolution pov.
But in essence, outside of solving the root cause, it's OK to just delete the contents of the temp/backup folder as its contents are not referenced anywhere in the database. That way I can get the disk usage back under control.
In the checksum folders have a backup.mbz file of 800+Mb, so it would point to the "copy" issue you speak of, and am on a hosted service, so hands somewhat tied.