Permacomputing

My typical backup system:

• a computer that needs backing up in on a network
• the computer hosting backups is on that network
• [optional] the backup host is either powered down, or has brought its network connection down (so it's not visible and not hackable)
• at a predetermined time, the backup host wakes (brings its network connection up)
• it checks if the backup source is present, aborting if not
• it logs in via SFTP (an FTP-like protocol built on top of SSH) with public key authentication and pulls the backups down from the source according to a script (SFTP can do a list of tasks basing on a command script)
• after successful download (but not after a failure to download) it searches for too old backups and erases them ¹
• finally the backup host powers down or leaves the network
• optional final step: occasionally, a disk image of the backup host is taken, the memory card is put in a bottle, the cap is screwed on tight and the bottle is hidden under a stone :)

Regarding data protection: ideally, both computers use disk encryption. Especially the backup host, since it's unattended and could be taken by a burglar (or a cop), and holds the private key that can access the backup source.

¹ erasing old stuff is easy enough in Linux/Bash:

for i in `seq 5 10`;
do
     DATE=$(date --date "$i days ago" +%Y%m%d)
     echo "Deleting backups from $i days ago, that is [$DATE]."
     # do something
done

...generates a sequence of past dates ranging from 5..10 days in the past, attempts to delete something for each. Or alternatively, for those who like fancier, shorter and a bit more risky commands...

find ${BACKUP_DIR}/backup*.tgz -mtime +10 -exec rm {} \;

...finds files in directory $BACKUP_DIR named "backup*.tgz" and if modification time is older than 10 days, passes them as arguments to "rm".