For last 3-4 days, Windows 10 Mobile users have been facing an issue that stops installation of Store apps to MicroSD Cards. Microsoft seems is aware of this issue and is actively working to fix it.
This issue may be fixed in the next Windows 10 Mobile build, it seems. As per Senior Program Manager, Brandon Microsoft may have fix for this issue in its next flight for Mobile.
We think we have a fix for this in our next flight for Mobile. Stay tuned.
— Brandon LeBlanc (@brandonleblanc) July 21, 2017