Move disks from Duo V2 to RN314

Hi, I was finally able to do the upgrade. Everything went fine, so thanks for all your suggestion.
As a reference, what I did is the following:
- Remove one disk from the old NAS
- Put it on the new NAS
- Boot as factory default
- Create the volume and copy everything from the old NAS to the new one
(here I had to change a bit the procedure because the original one was not working)
- Move the second disk from the old NAS to the new one
- Boot normally on the new NAS and enter to the administration page
- Switch off the X-RAID to be able to remove the volumes on the second disk
- Delete the three volumes detected on the second disk
- Enable again X-RAID
- The NAS automatically synchronizes the two disk

So, everything worked. The only doubt is that the volume is indicated as RAID1, even if X-RAID is enabled. I don't know if it is normal, but the two disks are synchronized, so for me it's fine.
Now I have another problem. I had two smaller disks, of 500GB (the other two are of 1TB), which I would like to use on the other two slots.
The two disks are formatted and, when I put them on the two free slots, simply nothing happens.
I can see from the administration page the two disks, correctly detected, but I cannot do anything on them.
Any idea? I would like to have those two disk configured as a new volume, also in RAID1 (or X-RAID2 or whatever), I mean, I don't want to add this space to the other disks, I just want to see them as a new disk. Can it be done? Do I have to turn off X-RAID again and create manually a new volume?
Thanks for any suggestion!