B&H has the WD 6TB My Book Duo RAID drive for $279 (reg. $349).
To get the special price, you have to click the “Email me a better price” link on the B&H product page. Then, B&H will give you a link that is only good for a limited time with the discounted price.
Check it out while it lasts here at B&H Photo.
These are very fast and affordable external drives. See my recent review of the 8TB version of this drive.