Hi, Even if you replace the current Drives with 4TB drives (RED, GREEN or whatever colour of the Rainbow you desire), the Capacity that you will end up with is the same as what the Drives are that cameout of the device originally (mine had 2TB Green drives)
I tried to do this and got 2x 4TB RED NAS drives in and working, even showed up as 4TB (Mine was in RAID 1), but when I went to the Shares Menu it only reported 2TB, when you go back to the Initial screen it now reports 2TB. Under storage the drives show as 4TB drives.
I logged a Case Question re this. Answer was not satisfactory! Apparently the OS is written on the Drives themselves and you need to be able to change the Capacity there, otherwise it just get copied from one drive to the next if you replace a drive at a time, as you cannot replace both drives at once.
If anyone has managed to do this please help us all out. I bought the 4TB model as the 0TB model did not exist here. WD just invited me to buy an EX2 diskless model to do what I wanted re the disks. However, suppliers here (in N.Z. and Australia) do not sell Diskless versions. Good luck.