Think about using rsync for back up copies

rather than making a recent copy and then deleting an old copy, we could:
- check there are enough old versions, if not just do a copy as usual, but if there are:
- rename the old copy to `rsync_target`
- rsync the current version to `rsync_target`
- rename `rsync_target` to the current date/time and then use that as the new copy

The rsync should be a lot faster than the copy.
