Ext. SD access fails for media, Galaxy s5, Marshmellow

[AC Question]

Probably not a new issue on this forum but maybe someone will direct me to a post/solution that actually applies and works:

I have 16 GB internal and 32 GB external storage.

Apps will move to my Ext SD and work fine.
Apps can't access media on Ext SD
Wifi Transfer Pro indicates that my Ext, SD is not writable
How can I get pics and music stored accessibly on my Ext SD!
Extremely frustrated
Bill