Maximum capacity at that time of sales is 2tb. mine was just 1tb when purchased but funny thing was I tried upgrading to 1.5tb X 4 it works fine for couple of weeks then suddenly, the unit stop responding & shuts down. Replaced all 4X hdds again and ran tftp procedure and same thing happen. Installed back the old 250gb and everything back to normal. Did a lot of tests still can't figure out what actually happen. Anyway now using as 2tb as it's the max it can support.