When I bought my NAS last year, I only put four drives in it, leaving four empty bays. I was planning on building a second RAID 5 array with 6TB drives, but NewEgg had a Cyber Monday sale on 4TB HGST NAS drives that I just couldn't say no to. I guess I will just have to make do with 21TB of usable NAS space. I'm sure I will manage, somehow.