Greetings @MariusM,
Can you write to your SD card from the terminal outside of your application?
Also it sounds like you’re removing/re-inserting the SD card while the container is running. In such cases Docker needs special options to handle the propagation of these events to the container. Otherwise you end up with situations as you described where the SD card is removed but still “appears” in the container.
There’s a bit more detail on this post here: Automatic mounting of sdcard on boot
Please try the bind-propagation as described in the other post.
Best Regards,
Jeremias