This works. BUT...
I can't find the file "Fichier2.txt". If I understood it is supposed to be saved
in the app directory. I can't find that directory either !!!
I did a "search" from the root directory and from the SD card using "FileManager"... No luck.
Yes, I read that File.DirInternal wont be accessible only by other app but I
thought that it could be possible using a file explorer... And realized that
the file explorer... Is an (another) application too.
I have had a look in my smartphone, I never noticed that none of the
applications (and their directory) I have installed are visible.
Probably a question of account security level (?)
If you run and debug your app using the emulator, I think you can access the folder by using DDMS and its basic file-explorer which permits you to pull and push files between the emulator and your PC.