Reduce number of disks... now that 3TB are available

Let me throw one more configuration option into the ring. This will be slightly more powerful - giving you more space AND doing a factory reset. You may have to play with the math to "juggle" the last 1TB of data, but it should be obvious.

1) Back up 3 TB of data to #1 of your new 3TB drives in a USB caddy.
2) Back up remaining 2 TB of data to #2 of your 3TB drives
3) Do a firmware upgrade (if applicable), power off.
4) Remove all drives. Remove all drives. Insert BOTH 2TB drives as #1 & #2
5) Do a factory reset. You now have a redundant 2TB array.
6) Add #3 (not used) 3TB drive as #3. You now have a 4TB (redundant, not using last GB of drive #3)
6) Copy 2 TB of data from #2 3TB drive to array
7) Add #2 3TB to array. Let it expand. You now have 2x2TB and 2x3TB. 1 3TB is "redundant", giving you 7TB.
8) Copy final 3TB from #1 3TB drive to array. You now have full 5 TB of data on a happy & redundant 7TB array.
9) Insert #1 3TB drive into array, giving you 10TB of "factory reset" array.

>>>>>>>>>>>> Alternative: Do not use your 2nd 2TB drive in this process. You can still juggle data - but in the end, you will have 3x3TB and 1x2TB, giving you 7TB of space and leaving the 2nd 2TB drive as "backup". However, you can instead use your "small" 1TB drives as 3TB of critical offline backup instead.

Now, you have so many options, you'll probably change your mind at least 4 times before you decide what to FINALLY do :) :)

*edit* Believe it or not, you can later add the 2nd 2TB drive to your array, even if it is not the biggest, and it will accept it. I originally had 2x1TB and 3x1.5TB. I switched to 2x1TB, 1x1.5TB and 2x2TB. Then, I swapped a 1TB or 1.5 without an issue - so I went to 2x2TB, 2x1.5TB, 1x1TB. The X-RAID2 is amazingly resourceful.