I was stuck for months on this trying to write the RootCA onto "3" which is the second system on simcom 7080G flash memory. Now I have an issue of convert file to the system. That's why I came here to seek for solution. Thanks for all comments above. I would like to share if this would help. I don't think merely using AT+CFSWFILE=3 can upload the RootCA to "customers" directory. I have just found a few days ago that it may need to configure the simcom 7080G before connect to the MCU (more easy) by using either USB. Before doing, I think it is a good idea to set internal clock to current because the default in simcom7080 is back to 1980 which all RootCA expires already. After AT+CFSWFILE=3 with correct parameters then you can see response from the simcom7080G with "download". You have to open the RootCA file, copy the content of file ready to paste into the terminal click send within the time limit. In your command is 500 millisecond which is too short, you can set max at 10000 ms (10 s). The file will upload with response OK. The file prepare you need to check the size exactly in byte using with line feed or carriage return or any special characters are not allowed. To make sure I use Notepad++. On the bottom right corner, right click the format to unix(LF) and UTF-8 only. After doing this you can use AT+CFSRFILE to read the file that uploaded to "3" or "customers" directory. I am struggling right after this to convert the file in to the simcom 7080G system. Any comment is welcome.