Not what RAID is for and you are in fact treading a fine line and for two reasons, you should never ever have any back up on the same drive because should the drive fail you have not only lost your primary data but also your back up, second reason, Seagate were at one time up there with Western Digital when it came to quality but with the advent of the Barracuda range quality suffered and so much so that any drive other than Seagate would do, want to know just how bad things got for Seagate, take a look here