The WJ-HXE400 is the extension unit for the recorder (WJ-NX400K, WJ-NX300K) which is sold separately.
The unit can be mounted with a maximum of 9 HDD units (hard disk drive installed in a canister).
They can be used as RAID (Redundant Arrays of Inexpensive Disk) to improve fault tolerance of the HDD.
The HDD units are easily mounted and dismounted on the unit by opening the front cover of the unit.
Make sure to connect a corresponding recorder to the unit because the HDD units cannot be set with the unit only.