* 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, and automate if possible; 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.
