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