HI
I have the same problem.
On SM G900F with MM 6.0.1 stock (XEF) (Rooted + TWRP)
The SD Card is locked to "read only" whith some apps (Titanium backup, Locus pro), I can't enable read/white.
Difficult to SD write acces with ES File explorer (root)
Have you 1 idea?
Thanks