A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
From Dahua Wiki
NVR Randomly Rebooting or Shutting Down
Issue: NVR Randomly Rebooting or Shutting Down
- Disconnect HDD and boot NVR. If NVR boots without HDD, then HDD has failed or is not compatible.
There are following possibilities: Explanation:
1. If Input voltage is not stable or it is too low, then change the power supply and cable.
2. If HDD malfunction or something wrong with the ribbon, then replace the HDD or the ribbon.
3. Button power is not enough, then add more button power.
4. If Front video signal is not stable then check and adjust front video signal.
5. If Working environment is too harsh, too much dust then clean it.
6. If Hardware malfunction then replace hardware.