When I used Sierra my screen saver was based on Aperture albums stored in an external drive. When I traveled the screen saver switched the default, but when I reconnected it went back to Aperture.
Mojave can’t use Aperture/iPhoto albums. So Screen Saver points to a share drive. When I travel it switches to default, but when I return it *doesn’t* switch back to my share.
Anyone have a workaround? I wonder about creating some kind of local alias for the remote image folder ...
Update: I created a symlink for the share folder that has my slideshow images using the SymbolicLinker service. Then I pointed ScreenSaver slide show to the symlink. When disconnected ScreenSaver shows black screen. When connected shows my images.