Depending upon the situation it can make sense to just take a picture of the whiteboard with a camera (or use a smartboard) and save that image somewhere. At that point the diagram isn't 'editable' per-say, but that can still be great solution.
I want to avoid the situation where the left hand doesn't know what the right hand is doing. Any person on the team doesn't have to go hunt down the person who knows the system, but can just look up the diagram at a high level and know where to start.