How do I move photos from the Mac to an external drive

You just drag the Photos Library from your Pictures folder to the Seagate drive and double click on it. Once you're sure that everything works (no reason it won't,) then you can delete the local copy.

But I've got to warn you: To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

Usually APFS is used more for Solid State Drives, and Mac OS Extended (Journaled) is used with mechanical drives.

That article also suggests that you check the permissions for your new drive once you're sure it's properly formatted. This is not usually a problem, but it sometimes comes up. 

A Photos Library may seem to work on a non-Mac formatted drive, but problems gradually appear later. If you want to double check using the Disk Utility, see this:

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Formatting the drive properly is usually the only glitch people run into. Let us know how it goes....

What kind of Seagate drive are you planning to use? Do not eve think of using a NAS or any drive accessed over your local network, if you want to use the Photos.app. The external drive needs a wired connection and must not have any syncing software or backup software installed. Be sure to get yourself a simple external storage, not drive that will try to mange your photos on its own.

treckymom2 wrote: ... Sometimes the most obvious is indeed the best answer

Unfortunately, sometimes what seems obvious turns out to kick you in the pants! It never hurts to check!

You're welcome...