Why do some registrars have the ability to insert 2 HDDs?

Question

CCTV

Why do some registrars have the ability to insert 2 HDDs?
For example here - www.hi.watch/product/ds_n316_2
One spare?
Or simultaneous recording on two discs if there are many cameras?

Answer 1

To have more space.

Answer 2

1. Inserted two disks, then took out one and took it somewhere to throw off the info. As long as you wear it, work goes on.
2. Fault tolerance in case of failure
3. More space.

Answer 3

4. To the sounded plus. The bandwidth of one screw (performance) may not be enough for recording / playback at the same time.