I have two S5 phones, and I don't think I ever tried to merge the extSDcard as internal memory.
I don't believe it was ever set up to do that.
But, leave you with a thought: that is a horrible idea right from the get-go....
If for some reason, the phone itself is damaged, EVERYTHING on the SD card is lost.
Completely lost:
why? all of the directory structure depends on the phone itself to keep things straight.
No other phone, even another S5, can recognize anything stored on that SD card.
So, I suggest you forget about merging the memories of the phone and the SD card.....
Horrible implementation, and whoever thought the idea up, should be shot at sunrise.
At least, with an extSDcard, if the phone dies, any phone can see the current SD card you have now.
Or, a PC for that matter.