I have 4TB Red Pros in use in my NAS, they work well. Another thing to consider would be large capacity Gold drives, if you find them on discount HDD sites, depending on price. I recall I found some 14TB Golds on one site, for about $120 each, but decided not to spend the money yet as I would need 5 of them to replace my current drives, and I don't need all that much space yet.