The new firmware for A7M3, A7R3 and A9 released by Sony fix the incompatibility with SanDisk SD Card.

Since Sony updated its A7M3 and A7R3 Ver.2.0 firmware in early October last year, then the Sony camera was incompatible with the SanDisk Extreme Pro 128GB SD card after the update, this situation was revealed on social media and later affected the flagship camera A9. It was more than two months after the mess; Sony finally updated the firmware for those three cameras to fix the incompatible issues with the SD card.

There is an end for the Incompatibility issue of SD Cards that happened over two months.

Following the release of Sony camera for the A7M3 and A7R3 Ver 2.0 firmware in early October 2017, a large number of users on social media at home and abroad disclosed that the Sony camera could not work with the SanDisk Extreme Pro 128GB SD card after updating the firmware. A similar issue occurred when camera A9 updated the Ver 4.1 firmware in mid-November. By early December, Sony had finally made his late respond, saying there was a problem with the new firmware, which was "deleted" from the official website and there would be a new firmware to fix the bug. The problem of incompatibility with third-party memory card issues was also reported in the domestic photography forum in China, social media users had commented that the SanDisk Extreme Pro 128GB SD card could be used after updating new firmware. Specific ways of fixing the new firmware of Sony cameras are as follows:

The solution for A7M3 Ver 2.1, A7R3 Ver 2.1 and A9 Ver 4.1

Taking one SD card that has been used many times, you can write data to this card under the RAW format, the camera may not work in a few cases. 
Please Note: 
When this problem occurs, it may result in abnormal image management, which will fail to correctly display the taken photos. 
In addition to stop the data that was being written at the time, the image data in the SD card is not corrupted or deleted. 
This way will resolve the compatibility issues with some third-party storage cards and improve the stability of camera.
The above ways can resolve the compatibility issues with some third-party SD cards. 

Regarding this issue, it was happening from the beginning of October to the beginning of December, the firmware caused the problem by the time of release and resolved the trouble by the time of offline. The reaction of Sony is extremely slow when they raise the solution, and user is still testing the reliability of the new firmware. One Japanese professional photographer was using the A9 normally, however, the camera automatically deleted two photos in the SD card, the data could not be repaired, and there was no mention of fixing similar issues in the A9 Ver 4.1 firmware.