What hard drive is needed for these NVR/DVR system? 

There are several requirements of hard disk drive (HDD)

1. The disk had better be the one used for survilliance, not for computer use.

Here is reason.

If you need buy a hard drive by yourself, please don't buy hard drive for desktop computer.

The hard drive for desktop has the word "desktop" in its description.

The hard drive for DVR NVR has the word "surveillance" in its description.

If you know the model of hard drive, search it in google to check it is desktop or surveillance use.

Below are reasons why desktop hard drive can not be used in DVR NVR.

A. The current to power on desktop hard drive is bigger than surveillance hard drive.

Therefore, if you install desktop hard drive into DVR NVR, DVR NVR may be not able to power on hard drive and make it run.

B. The desktop hard drive is designed for 8 hours everyday use, not for 24 hours every day use.

Therefore, if you install desktop hard drive into DVR NVR, hard drive may be broken after several weeks 24 hours running.

2. The type should be SATA. SATA2(3GB/s) or SATA3(6GB/s)is recommened. If you connect a USB stick or drive to USB port of DVR, DVR can back up video from internal hard disk drive to USB stick, but can't record to USB stick directly.

3. We recommend 5400 RPM.

4. size is 3.5”.

5. The capacity should be less than or equal to recorder's maximum capacity.

To know the max capacity of hard drive you can use on some models, you need to reach the support with the model of the NVR.