Buy.com, via Amazon sells the Cavalry EN-CAHDD2B-D CAHDD Series USB Dual-Bay Dock for $39.99 – $20 mail-in-rebate + $0 shipping = $19.99! This rebate works since it is shipped and sold from Buy.com. This dock has 2 slots for a 2.5-inch or 3.5-inch hard drive and supports up to 4TB of storage. There is also RAID 0, 1, 0+1 and JBOD mode.
Rebate here (expires: 9/21)