Very few systems support the use of multiple hard drives. There are only (2) current systems that Zmodo is producing that support multiple hard drives. They are the ZMD-DC-SBN6 and the ZMD-DH-SEN6-2TB.
The ZMD-DC-SBN6 can use two hard drives at once. You can install up to (2) 2 TB hard drives for a total storage of 4 TB's. The ZMD-DH-SEN6-2TB has a total of 4 SATA ports on the motherboard. Allowing for a total of (4) 2 TB hard drives and a total of 8 TB of storage.
The H9216UVDH which was sold from 2011 until Spring of 2012 supported up to four total Hard Drives--giving the DVR a total of 8TB of storage space. If your DVR unit is not the H9216UVDH model or the ZMD-DH-SEN6, then your DVR only supports the installation of one Hard Drive with a maximum of 2TB of storage. (The SKU and the model number can be found on the sticker on the bottom of the DVR)