I have 2x 2TB drives in Readynas Ultra 4 plus in X Raid2 --> giving me 1.86TB space I added a third drive 3TB and after some restriping --> I have 3.6TB from a total of 7TB
Yes. The full capacity of the 3TB disk can't be used till you add another 3TB disk. So if you were to add a second 3TB disk to the empty drive bay you would get roughly 2.7TB more space than you have now.
Remember disk manufacturers measure 1TB = 1000^4 Bytes whereas the NAS measures using TiB (like most PCs it uses TiB and calls it TB anyway) where 1TB(TiB) = 1024^4 Bytes. So a 2TB disk is roughly 1.8TiB and a 3TB disk is roughly 2.7TiB (note I'm being very approximate here you can get the exact number by doing e.g. 2 * 1000^4 / 1024^4)
So with 2x2TB + 1*3TB - 1*3TB you get roughly 1.8TB of space.
