Answer
WJ-NV200 Does Not Support HDD Expansion Units
The WJ-NV200 recorder does not have the extension slot connectors in the back for the Expansion Units to connect to. Only onboard storage is available, 2 x HDD slots in the main unit.
If Expansion Units are required, the WJ-NV300 recorder does have the capability to connect up to 2 x WJ-HDE400 units