2nd ReadyNAS DUO or ReadyNAS ultra

EDCUL wrote:
No point backing up to the Duo is there if it can only handle 2TB and I got 4TB of data to back up?

well of course the duo may not be able to backup your whole data set, but 2tb of backup is better than none, and since you already have the duo there is no cost. Selling the duo won't get you much.

I can get the ultra and put in 3x 2TB drives and have 2x 2TB volumes with reduntantcy? incase one of the volumes fail?

you should familiarize yourself with raid levels

3x 2tb can configured in 3 individual 2tb volumes, 1 large volume of ~6b (raid-0, non-redundant), 1 large volume of about ~4tb (raid-5, redundant), 1 volume raid-1 (~2tb redunant) plus 1 volume raid-0 (non-redunant).

The simplest and arguably best way is to use the default x-raid2, which you can start with a single drive, then add additional disks and get automatic redundancy and expansion.

Further, later on you can expand the array by replacing at least two of the 2tb drives with 3tb (or larger) drives and the array will expand to use the additional space provided by the larger drives.