Newegg.com has the WD MyBook external 3TB drive on sale for a limited time. It's a single drive with USB 3.0 and 2.0 and is priced cheaper than what you can purchase the drive for by itself as an internal drive. Although, it doesn't come with the RAID/Adapter card.
Check the reviews on it. Some folks don't seem to be very happy with it's failure rate. It's a tempting buy though. What do you think? Use it as an external and backup your shares to it or rip it out of the case and add as storage? Does the 3.0 TB size deter you with version 1 WHS? Don't forget the Promo Code!