Ok more fun with my new Moto X4. It was just upgraded from 7.x to 8.0 Android.
The off device storage option to an installed Micro SD device is no longer working.
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Please do Menu => About => Send Bug Report, so that I can see your log, which enumerates the available disk mounts.
Just sent. Hopefully it is the log you will find useful.
That log was useful. I just released an attempted fix for this issue into the most recent Beta Release, 10.2.46. If you get onto the Beta team, you can download this latest release and validate the fix.
I have filled out the beta tester form and not received any thing since then.