Amazon has the Western Digital Red 2TB NAS hard drive (WD20EFRX) on sale for $119.99. It ships free with Super Saver Shipping.
Western Digital Red 2 TB NAS Hard Drive: 3.5 Inch, SATA III, 64 MB Cache
WD Red hard drives are designed specifically for home and small office NAS systems. WD's exclusive NASware technology enables seamless integration, robust data protection and optimal performance for systems operating in NAS and RAID environments.