The external is a bridge only for games with files >4GB.
Many games don't have big files, and can be launched from the external without problem.
If the game folder start with an underscore (or is grey'd out in the manager), then there are split files inside, and you need to put it on internal.
There's no way, for the moment, to merge the files in real time (on disk access while playing). the files have to be full length before launching the game.