How do I properly back up my large Photos library with iCloud Photos and Time Machine?

There is no "Apple Endorsed" way. There is simply backing up to another disk. If you're not happy with Time Machine then try use an app like SuperDuper or CarbonCopyCloner, both excellent applications.

I use a MacBook, so I can't have an external drive permanently attached, so I I keep my Library on the internal drive and most other things on external drives. I use Time Machine about once a day. But In fact, I have two Time Machine drives, an SSD that's fast, and a mechanical drive that takes a while. Some days I do one or the other; most days I use both. Like muguy, I haven't seen or heard of problems with Time Machine backups.

In addition to using Time Machine, I periodically (maybe once a month) copy my entire Library to a separate external drive. Actually, I have several libraries, so I backup and copy all the active ones in this way. Those that don't change much don't get backups so often.

inAjet wrote: …But, unless your internal MacBook SSD is big enough to house your entire library with downloaded originals, your backup won't actually contain your original photos.

As I said, I have multiple Libraries. I don't want to share every picture on my iPad and iPhone, so my system Library is smaller. Many people with large Libraries do that. I was just explaining why I didn't keep my own System Library on an external drive. But you haven't told us why you don't just copy your Library to another drive from time to time.

There are many disadvantages to allowing a Photos Library to grow too large. As far as I can tell, most people with large Photos Libraries divide them up.

I don't think that your concern about Time Machine problems with Photos Libraries on iCloud. has been discussed here. If you've experienced this you should let Apple know here:

Feedback - Photos - Apple

By the way, referring to posts "above" or below makes no sense here, since we can order the posts in different ways.

inAjet wrote: …It seems the only way to truly back everything up properly will require at least some workaround that results in an 'un-Apple-like' manual workflow... which is exactly what I was trying to avoid... 😞

I'm not sure this is a big problem. The only pictures that are in limbo are new ones coming from a source other than your Mac. So iPhone pictures will loll about in iCloud purgatory for a while before getting to the computer for backup, but it's only the new ones, and only when they're not used.. But when pictures are exported, they must be downloaded first, and that's why léonie said "I am exporting regularly all new originals to folders." We need an extra backup for the originals, anyway. So that should take care of it.

Your setup seems perfectly fine. Many users with. larger libraries stored those externally and backup with Time Machine (or similar). What is problematic?

Your set-up should work, if Apple would fix a bug of iCloud Photos, that is making it risky.

Ideally, your local copy of your iCloud Photos Library on your external drive should be complete and contain all original image files, as you are not using "Optimize Mac Storage" for your Photos Library, but "download originals".

The hitch is, that Photos is dragging its feet about downloading the originals from iCloud. There is no status indicator telling us, if the download is complete. Most of the time the status bar beneath Photos is showing "Synced just now", but that does not mean, that all originals have been downloaded, only that Photos has compared the local library with iCloud. We can only be sure, that the library is complete and all originals have been downloaded, if we have been opening each photo to force a download. That is clearly not feasible for a large library. Photos 4 on macOS 10.14 Mojave has been the last version of Photos, where we had a command to force a download from iCloud for selected items in Photos. Since macOS 10.15 Catalina we are left in the dark about the state of the download.

I have tested several time the state of my local copy of the Photos Library, after it has shown "synced just now". There are several ways to test it:

• We can copy the Photos library to an external drive and then open it in Photos, without making it the iCloud Photos Library. When we get a warning, that incomplete items will be removed, we know, that the library is not yet complete. My library with roughly 80000 items usually had approximately 20 items missing, when I compared the number of items in the copy of the library to the number of items in the original library.
• We can make a copy of the library with the most recent versions of PowerPhotos. This will create a list of incomplete items, so we can force a download for the incomplete items, then copy again, until no items are missing.
• I have tested several time to restore the library from Time Machine, and the restored version had also missing items, unless used PowerPhotos to check for incomplete items before making the backup.

My backup strategy is to use Time Machine to backup my local copy of the Photos Library, but additionally I am exporting regularly all new originals to folders, so I am having a local archive of originals and recover missing items for the occasional incomplete original.

And my iCloud Photos Library is also syncing to another Mac that is also backing up to Time Machine.