[HowTo] RetroPie and OSMC: Retrosmc - Retrogaming on OSMC

So here the steps I would suggest (sorry for the late reply):

Copy the roms folder to your hdd:

cp -r /home/osmc/RetroPie/roms /media/YOURDISKNAME/

Unlink the romfolder in home:

unlink /home/osmc/RetroPie/roms

delete the real romfolder:

rm -r /opt/retrosmc/home/pi/RetroPie/roms/*

create a new symlink:

ln -s /media/YOURDISKNAME/roms /opt/retrosmc/home/pi/RetroPie/roms