Newegg offers the Western Digital 6TB Red Serial ATA 6Gb/s 3.5" Internal Hard Drive 2-Pack, model no. WD60EFRX, for $449.99 with free shipping. That's $5 under the per-unit price we saw last month and the best deal we've seen. (It's also a current low for this quantity by $44.) It's designed for NAS systems and features a 64MB cache.