There are two flavors of the My Book Live Duo, including a $399 version with 4 TB of storage and a $500 version with 6 TB. Both ship in a RAID 1 set-up which means all the data on one drive is mirrored on the other for an automatic redundant backup. That means you'll only be able to use about half of the number you see on the box, but if one of the drives should fail, all your data will still be safe and sound on the other one. If you want, you can switch up the configuration to get at all of that hard drive space, but you'll lose the safetfy factor.