How do I play slide show from an external hard drive on a Mac Pro

Is your slideshow a collection of image files in folder on your external drive?

The easiest way to see them as a slideshow is to select this folder as a source for the screensaver in the system Preferences > Desktop&Screensaver and then to start the screensaver.

• In the System Preferences > Desktop&Screensaver select a template for the Slideshow, for example Ken Burns.
• Then click "Source" and select the folder with your photos to show.
• Close the preferences and start the screen saver by hitting a hot corner. You set the hot corners in the Desktop and screensaver preferences.

You can also start a sideshow from Photos (from an album or a slideshow project), but then you would have to import the photos to your Photos Library.