No experience with the SS-839 (nor any SS-x39). Potentially an (e)SATA contoller not able to support drives larger than 2 TB, similar all earlier TS-509?
Give a try to remove all existing partitions (ie. from a Windows or IOS X system) and use the USB connection. If that does still not unveil the full capacity, some 64-bit feature required have not made it into the SS-x39 builds, potentially because of there was no need as the internal capacity can hadly exceed 16 TB.