I don't know what happened with the other thread...
It seems as though Apple hates keyboard shortcuts, like pressing D for Don't Save in dialogue boxes, and they've been adamant on removing this feature from pro apps as well.
This ticks me off,and all I can say is why?!?....
So I found another way around this annoyance that may not be news to some of you, so please, no timelines.
In the Keyboard System Preferences, look at Full Keyboard Access, then switch from 'Text boxes and lists only' to 'All Controls'. Exit the pane.
Now when confronted with a Don't Save/Save/Cancel, you can tab through the buttons and activate the one you want with the spacebar.