End of 2017:
Same question mark in folder appears again when firing-up the machine.
Assuming the SSD was faulty, I swapped it against the original HD and Boom, the machine works fine again, well much slower but still.
Since it has a 5 year guaranty, I brought the SSD to a Samsung Service Center (Bangkok, Thailand) and 10 days after, they declared the SSD is actually fine and might just have needed a firmware update. Ok why not after all...
They printed out many screenshots of the Samsung diagnostic software showing that all was in the green and one last shot showing the SSD up and running in a Mac environment connected via USB port (attached pictures)...
"USB port connected" and "internally on a MPB connected" is something quite different... anyways...