Answer
WJ-NX200 does not support HDD Expansion Units
The WJ-NX200 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-NX300 or WJ-NX400 recorders do have the capability to connect to the WJ-HXE400.
WJ-NX300 can connect up to 2 x WJ-HDE400 expansion units
WJ-NX400 can connect up to 5 x WJ-HDE400 expansion units