Adding (2) Additional Disk / RAID 0+1 Configuration

If you were to reconfigure your existing 2 drive volume as an X-Raid volume, initially it would be a 2TB (redundant) volume. You could then copy the data from one of the other 2TB drives (make sure you know which is which) thus filling the available space on the NAS. Then hot add that drive to the NV+ array and it will wipe the drive and then synchronize the drive to the other two drives. You will now have a 3 drive redundant array of 4TB (gross) with the x-Raid array emulating a Raid4 array. You can then copy the remaining 2TB of data off the second 2TB drive onto the NV+ (which will have a 4Tb redundant volume with 3 drives). When all the data is copied off of the last 2TB drive, it can then be hot added to the array. When it has been wiped and resynched to the array, you will now have a redundant volume of 6TB. You would have your 4TB of data and 2TB of space.

While using the gross volume of drives it is important to remember the 1000/1024 conversion to data storage. So a "2TB" will in reality only store 1.8TB of data and a "6TB" will in reality only be 5.4TB of space, but the fact remains that you will have more space with x-Raid than in the other options you listed.

You could use Flex-Raid to establish a Raid5 array with the same space, but I believe in that situation you have to install all four of the drive at the same time, which of course would mean you either have to buy 2 additional drives, or find another spot for your data.

I would also caution you to consider how you will backup your 4TB of data once you have it on your NAS. Remember, Raid, no matter which version is not a backup plan. It is only a convenience to protect you against a single drive failure. It will not protect you against the failure of two or more drives, disaster or failure of the NAS itself.