Hi!
I’m using RCM4300, and I need an example of how to write a .txt file on the SD Card!
Thanks! 
Look at the samples/filesystem/FAT/fat_create.c file and you will find a sample that creates a text file named HELLO.TXT and writes the string "Hello, world!
" to it. It then closes the file, re-opens it, and reads the contents and displays it to the stdio window.