The main problem is that I wanted to play from external HDD due to internal space problems.
Moving the game from external to internal worked but I wanted to know why
In the end the game that I downloaded had the original .psarc game files splitted using opensplit (that .666xx format) and that's...
I just registered to say that I have the same problem
Peace Walker working but MGS2 and MGS3 not working running from an USB
Tried the fix but it is still happening
I will try to move the game to the internal HDD when I have time