Backups of Photos

The simplest way is to drag your Photos Library from the Pictures folder to your external drive. This will create a copy of your library on the external drive.

Or select all photos in your Photos Library and use the command "File > Export" to write copies of your photos into a folder on your external drive.

Is your Photos Library an iCloud Photo Library with the option "Optimize Mac Storage" enabled? Then copying the library will not backup your Photos. In that case use the "File > Export" method.

Check the file system of your external drive, before you copythe Photos Library there. The file system needs to be MacOS Extended (Journaled).