Just a small remark. When an external disk for backup is not available, it will on MacOS (and probably other unix-like environments) not simply fail but instead create a new backup directory where you do not want it but then fail because that directory is not in sync with the local database. The new directory may (on MacOS ‘will’ — in case of automount) then prevent the disk to mount properly, leading to permanent failure of your backups. To prevent this from happening, some special configuration settings are available, see MacOS USB destination confusion (which should be instead called ‘non-permanent disk availability on non-Windows issues’.