It is another form of using hard drives as external drives. According to what I just read it can take any two drives and combine them into one. So I guess if you had 2 x 500GB it would show up as 1TB or a 250 + 640 would show as 890GB. I have something like this but it's a single drive dock. If it does what the ad says I may get one of these. http://www.buy.com/prod/cavalry-sata-2-bay-usb-2-0-dock/q/loc/57608/209842225.html