A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
Troubleshoot/NVR/Burn Error Troubleshoot
From Dahua Wiki
Burn error /USB back error.
1. If Burner and NVR are in the same data cable, then separate them.
2. If System uses too much CPU resources. Then please stop record first and then begin backup.
3. If Data amount exceeds backup device capacity which may result in burner error. Then please delete data.
4. If Backup device is not compatible, please change backup device.
5. If Backup device is damaged, please change backup device.