additional hard drive

[westgate_749916356645047_749916356645047]

I have a RLK8-520D4 with four cameras installed. The storage with the existing 2TB drive lasts only for 9 days.

Two questions, does this unit have a secondary spare HDD slot? 

Can non matching HDD work alongside each other? I see reolink articles state that some systems can use two 4TB side by side. Is that the case for RLK8-520D4? And can I add a 4TB drive to the existing 2TB drive? Or should I purchase two 4TB drives? 

Currently 9 days of storage. If I slot in a secondary 4TB drive, I would have a total of 27 days, does that sound about right? Also, do you have any tips adjusting the capture rate to significantly reduce storage space? Thanks! Andy ok more than two questions, but ...

[Crimp On_62210811129]

The product specifications seem to indicate that the internal hard drive can be up to 6 TB and there can be one external SATA drive that is up to 6TB.
https(colon, two slashes) reolink.com/product/rlk8-520d4/#specifications

There is a support article showing how to remove the internal hard drive and connect an external hard drive.

(Wanted to provide a link to the User Manual for this NVR, but I cannot find one.)