Backup to an internal drive on the spare machine, then set up a cron job to rsync from it to an external drive nightly.
You rsync to a new timestamped destination directory each night but use `--link-dest` on the previous night's backup so it hardlinks unchanged files; this gives you efficient revertable incremental backup.
Each week or so, swap the backup drive with the other one you keep offsite. If you don't trust your offsite, also encrypt the removables with luks.
Um, or so I've heard.
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