First of all. Thank you for answering.
Yes, the microSD card is already inserted and has FAT32 format.
What I don't understand is, why I don't I see neither the created folder 'etc' nor the copied file 'etc/rc.txt' when use ls /fs/microsd.
However I do see them on the PC. That's why I think, that I am missing some steps to include newly created folders and files on the SDcard on the nuttx file system.