Keep running backup to your external drive as is!
RAID 0+1 needs 4 drives. RAID 1+0 needs 2, but recommends 4.
Here is information overkill: Wikipedia
Quote: "RAID is not data backup." Says it right in the link! Only "saves" data encase of a drive failure...a point missed by alot of persons asking about RAID 1.
However, keep your RAID 0 array, this boosts speed. But keep backing up...