The Backups 321 Rule Illustrated04 Aug 2011
3 - Have three backups. So have a backup, and a backup of your backup. But surely, another is just madness? The first copy of your backup is just to save yourself from silly mistakes, like accidentally deleting your backup.
2 - Have your backups on at least two different media. Which makes sense. If all three of your backups are on the same disk, they aren’t truly fulfilling their purpose of the disk dies. So you want to have your backups on at least two different pieces of media. I would go so far to say two different types of media. If one is a disk, the other should be a tape, etc.
1 - Have one offsite copy. If your physical location burns down, how can you retrieve your backups, no matter what different types of media they were on