I picked up a SnapServer 520, which is a middle of the road NAS using SATA drives. I think it was $2500 for a 4 drive, 1tb device.

I was sold on the expandability. Simply add SANbloc S50 JBODs for up to 30 tb (or 60 tb? it's a little confusing). Now one NAS device can manage all my storage for many years to come.

Runs Guardian OS Linux but easily works in Windows, Unix, and other environments. Comes with BakBone Netvault to receive backups directly from clients (without going through a "media server"). It has an external SCSI port to send data to a tape drive or autoloader. Redundant power supplies and network cards, all in a 1U form factor (but it's really really long).

Good luck.