Thanks.
I understand, that I must open the Dashcam App, and then recording should start. It doesn't. There's no difference whether I open the Dashcam App triggered or manually. It does not start recording just by opening the Dashcam App. Should it?
The USB drive has been selected and it contains only 1 partition. If I open the Dashcam App and manually push the record button, then it starts recording, and the saving is properly done to this USB drive.