The latest My Book Duo features two WD Red 10 TB 7,200 RPM drives configured in a RAID 0, which allows for sequential read speeds of up to 360 MB/s. The new 20 TB version will be available on September 6 for US$800.
https://www.notebookcheck.net/Western-Digital-ups-capacities-for-the-My-Book-Duo-external-HDDs-to-20-TB.244260.0.html
"This might not be worth it, since two separate WD Red 10 TB drives cost around $560."
Please provide a link to where can buy these drive at $280 a piece as the cheapest one I can find is around $470.
Turn out it's only an auction and it's for the 5400 RPM version...
http://www.ebay.com/itm/Western-Digital-Red-WD100EFAX-NAS-Hard-Disk-Drive-10TB-/222620458069?epid=238512458&hash=item33d5369c55:g:wM8AAOSwRJ1Zm5Rk