The only redundant storage technique available that uses 3 drives is called RAID-5. This storage array allows one drive in your system to fail, while still retaining all of your data. The basic math behind Raid-5 is (# of Drives * GB per drive) – ( GB per one Drive) Raid can only use the maximum disk space for the smallest drive you have. In your case, a typical 3TB drive is 2794 GB. This plugged into the equation gives: (3 * 2794) – (2794) = 5588 GB of usable storage. (5.58 TB) Your 5TB drive will essentially waste it's extra 2TB and be used as a 3TB drive.