Ok, I've made a launcher in python to drop the drive from the rom section before to pass it to emulator.
It works great. I can connect my external hard drive to any machine and launch directly mala from it.
I have a portable mala and emulators !!
I've use it to correct another small problem :
In mess, for the megadrive, we have 3 machines (genesis, megadriv and megadrij) for 3 zones (usa, europe, japan).
I have one list for the three machines and my launcher choose the good machine to launch in mess.
Thank you for your answer.