Current Systems
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.
Old Systems
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)