If your hard drive can not be recognized/read the hard drive, please kindly check the HDD firstly.
(a). Open the cover of NVR. Uninstall and install HDD for several times.
(b). Check the wiring for the hard drive，check whether the sata data line is connected normal, remove the hard drive to check whether the internal wiring is connected to the DVR/NVR.
(c). Please check the power supply of the camera system. If the power is broken, the power will be insufficient, resulting in the hard disk unable to run.
If the issue isn't solved, please contact us at firstname.lastname@example.org