I have been going through the same problem for 3 months. That question could have been written by me!!!!
I was in Bangkok at first and was easily able to take it to someone who transferred. So what was I doing wrong?
Now I'm in India where no one is helping me. Not in a big city.
Ok here's 2 things I've learned by struggling with it.
When you put a new DD card into your phone it gives you 2 choices.
You can use it ONLY for transferring data between devices or you can use it for data storage.
I don't really understand what the first option is. I chose the 2nd option and filled the card with storage but couldn't transfer it.
Well the 2nd option doesn't allow you to use the card with ANY OTHER DEVICES.
I don't understand why they make these options. Seems silly to me.
So to even try to use the card in a card reader would not work.
I got fed up so many times and tried to remember what they did in Bangkok.
I think they DEBUGGED it.
That's an option in settings.
Under the category of Developer Options.
I've not fully got the solution yet but I'm starting to understand the reasons