Aloha h,
This may or may not work for you but our iMac 'puters are similar
so it might set you on the right track.
I had this same prob with Snow Leopard 10.6.8 and I what I found out
was it was caused by some 3rd party stuff (SampleTank or NI) still
trying to ‘hang on’ when I wanted to quit.
This usually happened when I was
1-working on a song,
2-hit save,
3-and then try to quit Cubase with the song still loaded.
So I started:
1-saving the current song
2-unloading the current saved song, (close the Project window)
3-open the ‘default’ or ‘empty’ Cubase song
and from there I could quit without crashes.
Might not be the answer to your prob but it is one approach.
HTH (hope this helps)
{‘-’}