I do this monthly bc I’m a lunatic and take hundreds of pics monthly of my pit bull and lab mutt. Haha
I plug the external hard drive into my MacBook air
An icon pops up on your screen ; open that
Make your folders (like 2020 - April pics) or however you want to group them.
Then I plug my iPhone in
Image capture pops up. Open that and all your pics are in order. Group highlight (*Command* then highlight to select as many as you want then hit *Command* again - that’s lets you select hundreds without tapping each one)
then at the drop down menu at the bottom it allows you to select the folder to attach to!