Finder in Catalina Doesn't Show Thumbnails When Searching SD Card or External Drive Finder in Catalina doesn't show thumbnails when searching external drives or sd cards. It is challenging to find what you are looking for when you have a lot of photos or videos to look through.  

What is needed to generate thumbnails on external media?
What I do is use the Cover Flow view of files in the Finder. Open a Finder window. See image below.

Here is a folder with 6 photos in it. Notice the red rectangle in the top left of image. That determines your view. If you use the 4th selection, you can see the images.