I have used the External Editors feature in the Photos App for several macOS Iterations. External Editors allows you to go into an external photo editing App from inside Photos, edit the photo in question and then save it back into Photos. I use it all the time with Luminar, Photolemur and other external editors. Since the initial release of Catalina, the external editor feature is broken.
The Catalina version of Photos has a lot of changes under the hood. I don’t know if the external editor problem is with the changes in the Photos App or possibly the External Editor App itself. The External Editor App does the following:
External Editors For Photos (EEFP) adds an extension to the OS X Photos application that allows you to edit your photos with any image editor on your Mac.
It really is cool the way it allows the special features of third party Apps to be used inside Photos. I think the problem is the External Editors App cannot communicate fully with Photos any longer because of the under the hood Photos changes. Here is what happens when I try to use a third party App to edit in Photos.
The third party editors area is accessed here:
When you click on that icon you get this drop down of third party editors:
In this case I am going to choose Luminar 3 to edit this photo. Once I am in Luminar 3 I apply the editing Filters of my choice. When I Save the photo this is the message I get:
This is the same message I get if I use Photolemur to do the editing:
The problem is always an error while saving.
This is a real bummer. I use third party Apps to edit photos all the time. For now I am putting off any editing to see if Apple fixes something in Photos or if the makers of the External Editors App fix something on their end.