Best way to backup 5TB of data on NAS

I'm basically in the same boat. I have > 4TB of used space on my ReadyNAS NVX. I would like the least expensive solution possible to have 2 backup sets of my data. One set will be colocated with the NAS and use rsync once a week or so to keep the backup current. The other (hopefully identical solution) will be done every 6 months or so and taken off site for storage. Maybe not the ideal schedule for everyone and every application but should be super conservative for my needs.

-Obviously the simple solution I have been using of a single 4TB external USB drive is no longer viable
-I do not want to have to own 3 NAS devices to meet this goal
-I strongly prefer not to split folders, manage multiple external HDs and the like
-I've tried a two drive bay, hardware RAID0, external drive enclosure but it is not recognized with the full capacity by my NAS (http://www.readynas.com/forum/viewtopic.php?f=24&t=73113)

This is getting a bit complex and frustrating. Sure lots of money will solve the problem immediately. I'm hoping there is another option.

Thanks in advance.