If I like to add new external midi device like GM or XG, system allow me to do this but during I make choice of instrument, Cubase quit unexpecedly and I loose all previous data. MAC OS 10.7 Lion is fresh after reinstallation.
Konstantin: Are you saving your Projects before
Recording anything 
If you have a CRASH Cubase will make a Backup.
But you have to save it first 
Jack 