Backup iCloud Photos with an Optimized Mac
The biggest problem with "Optimize Storage" on a Mac is that, since you don't keep the full sized copies on your Mac, you can't back them up with Time Machine or even by cloning to an external drive. You can't back up files that aren't there! On a Desktop, we can just use an external drive for the Photos Library, but for a portable MacBook, keeping a drive dangling off a port as we carry our Mac around is just too awkward!
So here's one solution-- make another user on your MacBook that uses an external drive for its Photos Library. You'll need to format the external drive in either APFS format or Mac OS Extended (Journaled) format, like is discussed here:
Move your Photos library to save space on your Mac - Apple Support
Then once a day or week or whatever, plug in the backup drive, and go to this second user and refresh its external Photos Library. Just turn iCloud off on that user when you switch back to your normal account, and switch the 2nd user's iCloud on when you go to it to refresh your backup.
Here are the steps:
- Plug in an external drive and format the disk in APFS or Mac OS Extended (Journaled) format as above.
- Make another user on your Mac.
- Connect User2 to your iCloud account.
- With User2, create a new Photos System Library on the External Drive, like this: Make a New System Library connected to iCloud- Apple Community and set it to Download Originals.
- Let the external drive fill up with all the pictures and information from iCloud Photos.
- Turn iCloud off, eject and disconnect the external drive, and switch back to your usual account
I say to switch iCloud on and off when you plug and unplug the drive, because that gives iCloud a chance to make a clean break, so to speak. It probably won't hurt if you forget, but it's better to do the switching.
When you are ready for another backup, just switch users, plug in the drive, and turn on iCloud. It might take all night for things to settle down, and then you can go do step 6 and get back to work.