Android - Using SD card as internal storage and migrate date
OK, I think I found out the best setup.
- Format your SD card as internal memory.
- Go to Settings > Storage > Internal and open each app. Some apps have a "change" button. Press it to move the app into the SD card.
- Finally go to Settings > Storage > Internal and on the right corner press Migrate data.
At this point, you did all you could.
You will need to format the SD as removable storage for it to work as you describe. The option as described by Dan is not available with your SD formatted for internal use.
I did read a post somewhere that suggests Google Play Music will have a fix this week but hard to say for sure.
In my opinion this feature either doesn't work as intended or the Google Play apps haven't been updated for this feature despite marshmallow being out for 6? months already.
Note: I have just installed the new update 22/4/2016 and you can now select the SD storage, so this is now fixed!