Every instruction manual contains a warning that Raid on the ReadyNAS is not a backup plan. Raid only protects against the failure of a single disk (except for dual redundancy on 6 bay units which protects against the failure of two drives). Backup to a separate device is HIGHLY RECOMMENDED. Never trust valuable data much less critical data to a single device. A proper backup would also include an off site device if feasible.
While many of us have trusted to the security of Raid in the early days, and survived, the failure of a single drive can bring on other problems. For example another drive may be developing problems that have not risen to the point of an alert, but the stress of a resync following the replacement of one drive can cause the failure of the second drive which then takes the data with it.
If you are copying large blocks of data that does not change rapidly, the periodic copying of these files onto large drives mounted or attached to a PC may serve well. Before I had a second NAS, I used this method to back up my video files on a periodic basis. I used a separate small WD Passport to back up my photo/financial files and stored the drive in my desk at work (safe deposit box now that I have retired). But absolutely nothing beats the ease and security of a NAS to NAS backup.
If you are looking at 12TB of data, you may want to consider an Ultra 6 or better populated with 3TB drives. With dual redundancy it would be close, but with single redundancy it would be a gross of 15TB (5x3TB with 1x3TB providing redundancy) and a net after overhead of just under 14TB.
Remember - the NAS is not a backup plan.
Backup = Good, No Backup = Bad