Western Digital has been making its My Book line of external hard drives for years now, but now the company has launched its first Thunderbolt-powered drives. The My Book Thunderbolt Duo (which we saw back at CES in January) comes in 4TB and 6TB varieties, but that high capacity will cost you: the 4TB model retails for $599.99, while the 6TB model sells for $699.99. Additional features include two Thunderbolt ports, so these drives can be easily daisy-chained. The "duo" in the name refers to the two separate hard disks in each unit (two 2TB or two 3TB) that can be run in a variety of configurations. RAID 0 mode splits data evenly across both disks, while RAID 1 mirrors the contents of one disk to the other; you can also have both drives show up on your desktop and use them as you would any other pair of external drives.

These drives should be quite speedy thanks to the Thunderbolt interface, though due to the limitations of hard disks, speeds will be far below the maximum 10 Gb / sec rate that Thunderbolt theoretically supports. Western Digital's page says that the 6TB model was tested at an average transfer rate of 250 MB / sec, with the 4TB model coming in at 225 MB / sec. For those that have been waiting for high capacity, high speed storage, this drive looks like a good alternative to LaCie's 2big Thunderbolt drives that we also saw at CES.