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Well that's how I thought it worked.
That isn't how it actually seems to work on my Moto X Play. It makes it look like there are two large storage drives (equivalent to an SSD and HDD I guess) and for most, but not all, apps you can choose which to use. I think this is how it is meant to work looking at this webpage https://motorola-global-portal.custhelp.com/app/answers/prod_answer_detail/a_id/109134/p/1449,9582 but the trouble is that some apps can't be moved to external storage and there is no longer an option to change where the storage is as everything is stored with the app.
The upside is that you can keep the most common apps on proper internal storage so they won't suffer from any slowdown.
After a bit more googling it seems this is a known problem with Google Music and a fix should be along shortly https://productforums.google.com/forum/#!topic/play/CgNiUtLjC1I;context-place=forum/play
Yes, but the moving of the apps is done by the OS which has awareness of the fact that the internal storage actually consists of two different devices.
The apps which run above that layer do not. They are simply presernted with a large, homogenous looking, internal storage.
Given that G Play Music has the impression of a large internal storage drive in which to store downloads, why does it matter that you can't tell it where to put them?