Transfer video and pics from Mac to USB

Get help with the Photos app on your Mac - Apple Support

The above link describes indirectly how to move an entire library to a different drive, and then to open that library again if the link become disconnected.

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.

I don't use Photos but I am pretty sure it is the same way as with iTunes and Music. Find your Photos library folder and drag copy it over to your external drive. I have no idea where this is kept except I would guess the folder is currently stored in your Photos folder just like iTunes is in my Music folder. Once you have copied it to the new locations, hold down the option key while opening Photos, when it asks for a library, select the folder you have put on another drive. It all goes well then you can delete the old library folder.

If you're worried about losing items then I am worried about your computer not being properly backed up.

Again like iTunes, if you forget to attach your external drive before starting Photos the application will default to using your internal drive. In that case quit Photos and start it up again while holding down option, etc.