For pre-existing engine installations (like external drive) the UE Launcher does not know and does not list them as being installed and it would be so much easier if you could simply add a “Use existing engine” type browse button and choose the folder/binary/whatever and let the launcher verify and add that to the existing engines rather than the current rename existing folder/pause/rename back work around as mentioned here https://answers.unrealengine.com/questions/584351/view.html