Our most recent Home Media Server upgrade saw us transition from a cludged arrangement involving a pacific rim NAS and external USB drives to a Buffalo Linkstation Duo (4TB). Content is backed up to two USB HDDs every month (a manual process), and we are now hovering at about 75% capacity (3TB).
The Linkstation Duo can handle twin 3TB HDDs instead of the 2TB HDDs presently installed, and you can hang a USB HDD (up to 2TB) capacity off the NAS.
That said I can’t help but find myself browsing through 4 and 5 bay NAS devices with up to 12TB of capacity (and a $700-1100 pricetag). I think I’ll spend some money on the 3TB HDDs, which can always be moved to a larger NAS down the road.