The safest way to back up an offline Photos Library

PowerPhotos is a ver good addition to Photos. And it can sometimes rebuild a damaged library, even if Photos can no longer work with it. It is the successor to the good old iPhoto Library Manager, that we used in desparate situations to rescue a Photos or Aperture Library.

I am using PowerPhotos to incrementally backup my new photos into an archive library I am keeping on an external volume. It is working rather well:

• In my main library on my Mac I am creating a smart album with the rule "Date added is after ..." (the date of my last backup)
• Then I use PowerPhotos to copy the items from this album to the Archive Library.

First, Apple tell us never to use our Photos Library from a networked drive. The Library must be used from a directly connected drive formatted in either APFS format or Mac OS Extended (Journaled) format. Attempts to do otherwise may damage the Library. (A static backup can be stored on other drives.) It sounds like this is what your are doing.

And, every backup is a snapshot. The fact that the Library may be constantly changing doesn't impair the quality of the backup. It may be that a minute later you will have one more face identified, but the backup for that moment will be fine. We have not seen any evidence that a backup copy (and lots of people here make lots of backups!) has been corrupted because of background processes. I use Time Machine, Carbon Copy Cloner, and straight drag and drop-- they all work fine.

I know you have re-built the library. Have you tried starting in "Safe Mode" to see if the problem is is with your account, or with caches and login items, and stuff like that?

           Start up your Mac in safe mode - Apple Support 

One way to make a fresh copy of your library (though it will take weeks!) is to connect to iCloud, wait for it to synchronize, and then set up a new library and let it re-populate. It clears up a lot of sins. You can then disconnect from iCloud.

I do not use "Optimize Mac Storage", so I would expect all original image files always to be locally downloaded t my Mac's library, but that is not always the case. When you are seeing "Synced with iCloud just now" in the status bar below "All Photos" all changes are in sync, but it does not mean, that Photos has downloaded all original image files from iCloud, only the versions and albums and folders are in sync. There is simply no indicator that all originals have been downloaded.

I recommend to test your copy of the Photos Library on the external volume, if it is complete and contains all originals. When you open the backup copy of your library in Photos, you should be getting a warning about incomplete items, if not all originals have been downloaded, when the copy has been made.

I am using PowerPhotos to make backup copies of my Library. PowerPhotos will show a list of all incomplete items, that could not be copied. My library has currently a size of 57000 photos, and usually ten to twenty photos, that I have not used in a ling time, will have missing originals, that never have been downloaded from iCloud.