Transfer photos from macbook to external drive

To transfer all photos to an external drive, move the Photos Library.photoslibrary from the Pictures folder in your user home folder to a folder on your external drive. Then double click the library in its new location to open it in Photos and test it. The external drive needs to be compatible with Photos. Prepare it like described here: Move your Photos library to save space on your Mac - Apple Support

If you use want to save selected photos on your external drive and not all your photos, select the photos in Photos and use the command "File > Export > Export unmodified Originals" to save the high resolution originals on your external drive, or "File > Export > Export ... Photos" to save the edited versions, or use both modes to to save the originals and the edited versions.