WJ-NX400 (recorder) has four kinds of HDD operation mode as follow;
1. Single mode
In this mode, Recording camera images and sounds onto each HDD sequentially.
2. Mirroring mode (V2.0 or later)
The same images and audio from the cameras will be recorded on 2 HDDs like HDD1
and HDD2, HDD3 and HDD4 in this mode.
When one of the two HDDs is replaced with a new one, it is possible to copy data (Mirroring recovery)
on the other HDD to the replaced HDD.
This product or the extension unit requires two or more HDDs.
3. RAID5 mode
Even if one of the HDDs becomes faulty,
recorded images can be recovered using the other remaining HDDs in this mode.
This product or the extension unit requires three or more HDDs.
4. RAID6 mode
Even if two of the HDDs become faulty,
recorded images can be recovered using the other remaining HDDs in this mode.
This product or the extension unit requires four or more HDDs.