I'm running out of room for photos on my my Mac. I want to move them to an external drive, and have a 2nd external drive as back up. I am using Big Sur 11.2.3 and Lightroom Classic release 10.11 and Camera Raw 13.1.
All you have to do is move the top level parent folder that contains all the subfolders with photos, and tell Lightroom Classic where they went so it can reconnect to them. There’s a good explanation at the Lightroom Queen website:
In this case it’s probably better to use Option Two on that page.
For the backup, use any good Mac backup software that can automatically maintain a mirrored backup of one volume to another. Some of the most popular and reliable utilities for this are Chronosync, SuperDuper!, and Carbon Copy Cloner. All can quickly update a mirrored backup with new or changed files only.
My preferred explanation is that you COPY (not move) the photos in your operating system, keeping the folder hierarchy unchanged, then re-link the parent folder in Lightroom Classic. Then once you are convinced that works, you can delete the hpotos on the original disk, if you want.
Explanation, with screen captures, in "Part 2 — Updating Folder Location" of this document: