* Always back up in **some way**. While a copy of the data on the same array won't protect you against all problems, it will protect you against some.
* A **backup on the same server** is susceptable to the **same failures as the original data** set (hardware failure, natural disasters, and the like).
* A good rule of thumb is **three copies** (the RAID is only one copy for this purpose): the **original**, one **onsite copy**, and one **offsite copy**. Store the offsite copy in the cloud, or at a friend's house.
* **Make backups regularly**, at least once a week; the day you need a backup is the day you realize you hadn't run it in 6 months and what you need isn't backed up.
* **Test backups regularly**, at least once a month; **a backup is worthless if you can't restore from it**. Just because you have a backup doesn't mean you're protected; always test them.
There are dozens of backup utilities out there; I'm not going to prosthelytize for any one of them, but I personally use [BackupPC](http://backuppc.sourceforge.net/) for my server and workstation backups.
