I've filled several 2TB external hard drives backing up images, and now I'm working with 4TB drives
Just a question when you say several "backing up", are you doing all new backups each time, or are you using a Syncing app that compares your existing HD makes deletions and additions as required so it says in sync with the drive? I actually do both, I use Sync for the general backup, and then archive those things that I don't want to throw away but no longer need on a HD.
I have a folder that Lightroom uses for its catalog, named of course "Photo_folders_Lightroom" which contains all the folders of photos. There are folders by year, with subfolders by month and camera (3 Fuji cameras and 1 mobile phone).
To keep my Mac's hard drive clear, the main catalog folder is actually on an external hard drive. I import all photos from the SD cards using Lightroom to rename and organize into proper folders.
On the back up external hard drive there is a folder named Photo_folders_Lightroom_backup into which I put copies.
Since the Mac operating system is based on Unix, there is a command line app called Terminal.
In Terminal I use the rsync command to copy folders from the main catalog drive into the back up drive.
rysnc -avPh --delete
is the command I use, and rsync analyzes the folder and copies over only files that have been changed since last I had done this. The flags -a -v -P -h ensure that all the photo file attributes copy over, and the --delete flag deletes any photo files that I may have deleted from the original folder from the back up folder.
Here is what the command for copying looks like in practice:
rsync -avPh --delete /Volumes/4_TB_Blue_161103/Photo_folders_Lightroom/2019/2019-09_X100F /Volumes/4_TB_Black_190903/Photo_Folders_Lightroom_backup/2019
It's not as complicated as it seems as once the rsync -avPh --delete is written on the command line, Mac allows us to drag and drop the appropriate folders into the Terminal window and Terminal writes out the full thing.
So the above command copies anything that has changed in the 2019-09_X100F folder from the main catalog to the 2019 back up folder on the other external hard drive.
Clear as mud?
When starting a new back up external drive like I did last week, I start with the folder Photo_folders_Lightroom_backups and the subfolder of 2019. I'm only backing up folders from this month and going forward. The back ups on the back up drive I'm no longer using go up through August of 2019.