A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
Troubleshoot/NVR/Unable to Playback
From Dahua Wiki
Unable to Playback Recordings
- Check that HDD is detected by NVR.
- "Record Status" is set to, "Enabled" on all channels.
- Check, "Record Schedule" and ensure your desired recording is enabled; Motion, Schedule, or Alarm Recording.
- Recorded file has been overwritten.
- File is in DAV format. Use Conversion Tool.
There are following possibilities:
- If HDD ribbon is damaged then replace ribbon or HDD.
- If HDD is broken then replace HDD.
- If Upgraded program is not compatible then find another program to upgrade.
- If the recorded file has been overwritten then there's not much you can do unless you have backup.
- If Record function has been disabled then enable it.