How else you suppose data BACKUP be done if not over network or on a external device. RAID is not backup.
The MacPro does not provide the capability for an internal RAID configuration,
The MacPro does not offer the ability to backup to an internal device such as a DVD drive.
I really don't understand the importance of the distinction between internal and external. It's sitting on your desk for pete's sake, just plug stuff in.
It might well prove quicker to use an ordinary backup program (my 2 x 2TB RAID1 system takes about 10-15 hours to rebuild, while Acronis will back up the 1.2TB of used data in about 5 hours). And there's a bit of inconvenience, in that you have to shut your computer down and swap the drives. But in exchange, the backup is guaranteed to be atomically that of your computer in its shut-down state - at least on Windows, I think this is difficult to guarantee otherwise.