I have Zyxel NSA 221 (4.41 firmware ver.) with RAID1 and single data partition.
I wish to change 2 x 1.5 TB to 2 x 2 TB HDD for bigger space.
How to do that with keeping data and RAID1 configuration?
You would probably have to copy all the files to an external USB or your computer and then swap the drives, or you could try to mount the drives in a Linux box and then copy to the new drives.
However, why would you go from 1.5TB to 2TB and not go straight to 3TB (or 4TB). The extra 500GB will fill up quickly and then you will need to upgrade again.