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, 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. Uninstall and install HDD for several times.
3. Install hard drive and put hard drive close to your ear to check if you can hear some noise. One hour later, use 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 share us the result.
4. Unplug DVR or NVR from power and plug again.
5. Change an adapter of DVR.
6. Try a spare DVR or HDD if you have.