A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
From Dahua Wiki
IPC failed to recognize SD card.
- SD card might be loose. Which would case the IP camera not to read recognize the SD card. Check if the SD card is secure.
- Check if there is SD card connection pins. Some models may have the slot with no connection pins. If this is the case please refer to the specifications and place of purchase.
- Check if the SD card in the computer is recognizable and there is no abnormality.
- It is possible the SD card is bad. Try the SD card in other devices to see if it is recognized. It is possible the connections of the SD card are damaged. If this is the case, refer to the place of purchase.