best way to export all photos to external drive

You've a wonderful way of not answering a question ;)

I'll take a wild guess and assume you want to move your Phots library to an external disk. This means you will want to continue using Photos as before...

Make sure the drive is formatted Mac OS Extended (Journaled) or afps

1. Quit Photos

2. Copy the Photos Library from your Pictures Folder to the External Disk. You can do this by simply using drag and drop

3. Hold down the option (or alt) key while launching Photos. From the resulting menu select 'Choose Library' and navigate to the new location. From that point on this will be the default location of your library.

4. Test the library and when you're sure all is well, trash the one on your internal HD to free up space.

The external volume needs also to be locally mounted and must not have been used for Time Machine backups, see: Move your Photos library to save space on your Mac - Apple Support

The straightforward answer, assuming you want to export - which means the images will be moved out of Photos entirely - is to select the images and use the File -> Export command.

However some folks also use the term export when they mean move their library intact to an external drive? Is this was you mean?

If you just export, you lose the ability to go back to the original. If you export the unmodified, you will lose any edits.

There is no other way to export.

What are you hoping to end up with? Your images outside Photos entirely? Or remaining within?