Big Sur Photo issue with back up

Do you have tried to launch the Photos.app from the external drive? Photos is a part of the system. You need to open the Photos.app that has been installed by Big Sur.

• Open the folder Applications and find the Photos.app there.
• Hold down the options key ⌥ while double clicking the Photos.app. release the key, when the library chooser dialog appears.
• Now select your Photos Library. You need to find a backup of the Photos Library, not a backup of the Photos application. Look for an item that is a stack or a fan of three pictures with the rainbow flower.

Hi

Have you copied the photos app to the backup disk? If so that won't work, and in any case, the photos app doesnt hold the images inside the app.

Images are stored in a library (normally called "photos library.photoslibrary" and normally stored in your pictures folder. Do you have a backed up library on your external drive?