save photos to external drive

Get help with the Photos app on your Mac- https://support.apple.com/HT205966

The part below is modified from a section on this document.

Create a new Photos library (or open one that has been moved to a new drive):

1. Quit Photos.

2. Press and hold the Option key while you open Photos.

3. In the dialog that appears, click Create New.

4. Give your new library a name, then click OK. Note whether Photos opens successfully with this new library.

To open a different Photos library, quit Photos. Then open it again while holding the Option key, select your other library, and click Choose Library.

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support