2nd ReadyNAS DUO or ReadyNAS ultra

You might want to consider purchasing 4 drives. Install 3 and establish the 4 TB volume under X-Raid2. The fourth drive keep in a cabinet or closed as a spare or future expansion. Drives can and do fail. Most often a failure either occurs very early (I lost my first one within 30 days of installing my first NAS, it's replacement ran for over 3 years) or after reasonably long service. Nothing quite gets your attention like an e-mail from you NAS alerting you to a drive failure. Having a spare handy gives a lot of comfort. Then you can get the failed drive replaced or purchase another spare without having to rush it.