I've got Deja Dup (14.2) backing up to an external USB drive on Lucid.
Up until now, I've had "Keep Backups for" set to "Forever". However, after a couple of months of usage, the drive is now full.
I've tried setting the "Keep Backups for" setting to a shorter time period, even as low as one week, my assumption being that older backups would be cleared when the next backup runs. However this doesn't seem to happen and I get an error message along the lines of "cannot copy temp file x to backup location y" and the backup stops.
How come space isn't being made for the new backup?
Best Answer
It looks like Déjà Dup keeps at least two full backups before it will free up space by overwriting old ones. You mention you have been using it for a 'couple months'. I bet that means you are up against this limitation. Full backups can't be scheduled exactly, but it is going to be about once a month with scheduled backups happening weekly.
You can manually delete old full backups, they are all those 'duplicity-full*' files on the backup drive. Make sure you have at least one full backup to recover from, obviously. You are taking a risk here that your remaining full backups are not corrupted, too.
Best practice before deleting anything is to use another drive to make a tested, known good backup to, then try deleting and/or resizing the backup partition on the destination drive.
Since you know you normally need 2 backups before Déjà Dup will make the third (and overwrite older ones), you need a destination of roughly 3x your current quantity of data being backed up.
If you are seeing different behavior or any bugs, you can ask a question or report a bug on the project's Launchpad page (requires a Launchpad account)