I managed a JSON configuration file in order to read, modify and save parameters on the root storage of the camera.
This is working but seems the file is in cache and not flush on flash.
If I re-read the modified fields I get the modified values but the original file is not modified.
Answering myself. I have found explanation on the forum saying the OSes camera drivers for Win,Mac,Linux can’t be changed to refresh the USB storage. Needs to disconnect and reconnect camera to see the change or call pyb.hard_reset()