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 ...