Please refer to these below to solve the HDD not found issue.
1）If hard drive was preinstalled, there are 4 screws at the bottom of NVR box.
2) Check the power adapter connected to the NVR box. The NVR box uses 12V 2A power supply. If you connect the 12V 1A power supply to the NVR, the NVR box will not have enough power to drive the hard drive.
3) Please open the NVR with a screwdriver, check the two cables connecting the hard drive and see if they are loose.
4）Take the hard disk out and install it on your computer. Check if the computer can recognize the hard disk.
Check Hard Drive Information on NVR box and Format it
Right click the mouse>system setup>General setup>HDD setup>select the hard disk>Click Format