M66 module at commands example for reading, writing and appending file in SD card
To read /write /appending file to SD card , pls refer the below AT 
“Quectel_GSM_FILE_AT_Commands_Manual_V1.5.pdf”
https://cnquectel-my.sharepoint.com/:f:/g/personal/america-fae_quectel_com/EnLBscTou4FOubavKK_LrqgB5_sqHT1pfAiSH6XR2UJn_w?e=JP8iMT