How can I begin a Slide show of ,y pictures from an external disc

Do you want a slideshow that runs by itself or one that you toggle from one photo to the next?

If it's the latter then select the photos in the folder and use the key combination of Command+Space to start a Quicklook of the photos. You can advance or go backward using the Forward/Back arrows on the keyboard.

Or if you want the slideshow to progress at it's own speed select one of the Screensaver options that let you pick the folder of images to use and use the hot corner to start the slideshow:

OR use a 3r party slideshow app to create the slideshow from those image files.