I have a 4TB external Time Machine disk. There are three internal volumes totaling 3.3TB that have been actively backed up to that external drive for a few weeks.
I was troubleshooting something and moving a bunch of files around, so I briefly turned off Time Machine. When I turned it back on, it started backing up... but then it stopped later, saying "there is only 707MB available on the drive, but 3.3GB is required."
It's almost as if it refuses to overwrite the old backups. It wants to start an entirely new backup or something. How can I make it pick up where it left off without having to delete that entire backup and start anew? I don't like deleting backups!