@tera thank you for your answer!
...what I tried:
sudo mount -rw -t vfat /dev/sda /media/myfolder
...and after sucessfully mounting
sudo chmod -v -R 777 /media/myfolder
...all folders/ files in /media/myfolder were shown (in terminal!) as being changed from rwx rx rx ---> to rwx rwx rwx.
So, full of hope, I opened my phone's filemanager and tried to change a file's name in /media/myfolder...
...but neither adding a file nor changing its name was accepted by the system...no write access! Also, of course, the contents of files couldn't be changed by e.g. Seabass2-app.
In spite of the permission changes signaled by the terminal, the files on the SD-card obviously remained unchanged.
Had no success also when trying a mount-folder in /home/phablet/myfolder...
The files on the SD card were written by myself with the "external storage"-app on the same phone. So I should be the owner/ root of the files...
What's wrong with my trials?