As mentioned in other posts it is only possible to write pictures, videos and music to an external flash with Windows Devices (In my case a Raspberry Pi 3 B using Windows IoT).
Thankfull to this post I am able to write my data to a jpg. Mike Leonhardi wrote that he was able to write also a txt file by modifying the capabilities.
Can someone tell me which modifications have to be done?
Thank you very much!
Windows IoT Raspberry Pi 3 c# Create .txt file
Here you can find the modifications which have to be done.
Instead of .txt you can also use .csv to write a csv-file to your external flash.