1. Please take it out and plug it into your desktop, to check if your computer can recognize the hard drive or not. If the hard drive can be recognized by the desktop, please format it to Read&Write status then plug it back to DVR and check if the hard drive can work again.
2. Please change an adapter (with the same volt) to power the system(the unit) as this issue can be caused by an insufficient power supply
3. If you have other spare hard drives, please try to replace the hard drive with the new ones.
Base on the steps, we can identify if the hard drive problem or NVR problem.