@Paolo Cadeddu - Not sure I follow why you put the SSD into the optical drives bay instead of just swapping out your HDD for the SSD.
If you are still encountering the diagnostic error: 4HDD/11/40000000: SATA (0,0) then you haven't solved the root issue and going down the GPU path is a red herring and has nothing to do with your problems. In fact altering the Kext files may have messed you up more.
Can you give us some more history on what happened with your system. Did you have a liquid spill into to the keyboard or trackpad? Did you sit the system down on a wet surface or drop your system?