Can I add a 5th Disk?

Papabear - My volume size is 8326. I am using about 51% of this, so a touch over 4 GB is free. I do use an NV+ to backup the important data on my system, but I have a lot of unimportant stuff I'd rather not lose that I don't backup, because I have no easy way of doing so. In other words, about 3GB of my data is not "mission critical" but would be nice not to lose, and this data is not backup up. Thus, I'd rather not do a factory default.

Leia: When I set up my system a year and a half ago, I added the first three 3TB disks one at a time, followed by a 4th disk which I added recently. The first disk I added gave me redundancy, and the second two disks gave me more space. I have already used about 5500GB of my 8 GB allowed expansion. The 8 TB expansion limit was not well publicized, and the system documentation for the Readynas suggested that doing things one disk at a time was the way to go. Obvously, this was a mistake, but I didn't know any better at the time. I want to know if I can safely add another 3TB drive, and get 2400GB or 2500GB of expansion.

Thanks!