Photos library File backup

iCloud Drive is different from iCloud Photos. (I think this is due to the history of their creation, not just to be confusing.) iCloud Drive acts like an extension of your Mac's drive. If you drag a file from your drive to iCloud Drive, it disappears from your local drive. You can use iCloud Drive as a backup by dragging copies of files there. (Oddly, the Photos Library behaves differently with iCloud Drive. Dragging your Photos Library to iCloud Drive is just a pretense-- it really stays where it was, and does not locate to the cloud. For the cloud, Photos is dealt with differently.)

iCloud Photos is not for backup. What it does is it makes all your pictures available on all your devices. When you turn on iCloud in Photos and on your iPhone and iPad, then the pictures are copied to iCloud Photos (where you can view them in your iCloud account on the internet) and then distributed to synchronize Photos on all our devices. You can't rely on iCloud Photos as a backup, because if you delete a picture on any device, it is then quickly deleted on all of them. No backup.

I keep two Photos Libraries (actually more than two.) I have the main library which is local only, and then I have a favorites library that I synchronize with iCloud. Since it has only favorites and recent pictures on it, it's smaller. I copy the favorites from the main library to the Favorites, so if something gets deleted from Favorites, it's OK. The main library is not connected to iCloud Photos, so it always has the full sized images. That's the one I back up by copying to an extra hard drive. If this main library is crazy big, lots of people keep it on an external drive (not the backup drive!) and just plug it in to work with the pictures. Then we copy the favorites over to the Favorites library. iPhone pictures automatically go to the Favorites library, and so you copy them to the main library.

I hope this is a little clearer and gives you some ideas...

Are you using iCloud to synchronize your Photos? If yes, are you using "Optimize Storage?" If you optimize storage, then each copy you save may not have full sized images-- those may be loaded to your computer only when you need full size to edit or print, etc. So you may be backing up only smaller sized images, not the real things. In that case, you should turn off Optimize and backup after the full sized images are restored. (When that happens is pretty iffy.) Sorry to start with this if you're not using iCloud. But if you are, you might need to ask about that iffy part.

If you're not using iCloud, then the difference between old backups and new backups will be twofold: one difference will be any pictures you've deleted. That's a point with backups, after all, is in case you mess up and delete something you're sorry about later. So after awhile, when you're pretty sure you won't regret it, you can delete the old libraries.

The second difference is the MacOS that was used with the old libraries. For each new version of MacOS, the Photos database changes, so that a library from new Ventura can't be read by a Mac using older Monterey. So, if you're sure you don't have any computers around that you want to keep on older OSs, then you can delete the old libraries.

Frankly, I've gotten burned a couple of times from crashed hard drives, so I keep everything-- and I have way to many old hard drives.

Yeah-- try some things out. You can create a new Photos Library by option-clicking on the Photos app icon, and selecting "create new." Then you can throw in a few pictures and mess around with it to see what happens. You can switch back to another one by double-clicking on another Library. By default, Photos Libraries are in the Pictures Folder.