Does Cubase give you this message when you quit or do you only get it when opening Task Manager and click on “Cubase not responding” after trying to quit Cubase? If the latter, then wait instead. Make a coffee, take the dog for a walk. Chances are it will close eventually, though that isn’t set in stone. Not ideal but force closing corrupts my sound card drivers causing a reboot to get them back. Your mileage might vary.
This is a long term, on going problem that only occurs with Cubase. Search the Forum for “Cubase hangs on exit” or look on the miscellaneous forum for a quite recent one.
Also, out of interest, try this. When you’ve finished your session, save your file. Then make a change. Move a clip or something. Then go to File/Quit. Choose, Don’t Save. See if that makes any difference. Sometimes does and sometimes not.
Sorry to report but there doesn’t appear to be a clear answer to this problem and because it only affects a minority, Steinberg don’t appear too interested.
And as a side note. I still get this issue even after reinstalling Windows 7 onto a brand new SSD, though the method above has improved things for me. Further more, this is a cross platform problem for both MAC and PC.
The only common factor is the dongle.
Good luck.