RAID1 is not for backup its for redundancy incase of hardrive failure. It should not replace full backups. You want to backup incase you or a customer deletes a file on accident and you need to restore it, or maybe if a hacker gained access to your system and vandelized the machine by removing files or replacing them.
RAID1 + a full backup of the system to a seperate server would be the ideal setup.
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It depends on what you are trying to achieve. RAID does not replace proper backups although it can complement it as part of the total backup solution.
RAID 1 basically ensures that if your primary harddisk fails, your server can in a way ensure uptime during the failure. Since RAID does mirroring, it means that any files removed or modified on the primary drive would be reflected on the mirrored drive. This means that if you or your customer removed or replace certain files and require a restore, you will not be able to achieve it. A bad update or a vulnerability attack would not be protected by the RAID solution.
Like Ameen said, a better solution would be RAID + backup and preferably to a remote harddisk at a remote location so that you know your data would be secured.
If you have to choose between RAID or Full Backup, I'll take Full Backup anytime over RAID.