In some latter versions of Android things apparently changed as to how apps are allowed to write on an external sd card (they aren’t).
If some app gives you Not writable and doesn’t properly asks for permission to do so, do the following.
Edit the file /data/system/packages.xml and look for the app name in there. In it’s perms part, add the following:
<item name="android.permission.WRITE_MEDIA_STORAGE" granted="true" flags="0" />
Reboot for the change to take effect. Solved the problem at least in my case for clementine remote and Android 6.