Go to System Setup-->General setup-->HDD setup to check the status of the hard drive. If it says Formatted, it means it is working. If it is Unformatted, please format it.
If DVR or NVR can't recognize HDD or cannot format, we can try methods below.
1. If the hard drive is not from us, please make sure your hard drive is for surveillance use, not for PC use,
2. Open the cover of DVR/NVR. take out and install the hard drive again. Hers is the method: https://support.sannce.com/hc/en-us/articles/900000433303-
3. Put the hard drive close to your ear to check if you can hear some noise. One hour later, use your hand to touch the hard drive to check if it is warm. If you can not hear any sound and surface of the hard drive is not warm, it can prove hard drive is not even powered up. Please try to change another power adapter to connect the NVR.
4. Unplug DVR/NVR from power and plug it again.
5. Try another DVR/NVR or HDD if you have.