macOS Photos: Optimized Storage version and External Drive Full Resolution version of same library

If your full resolution iPhotos Library won't fit on your internal drive any more you can switch to optimized storage. Even for a large library that's only 30GB or so [1], But they you can't do true backups with full resolution images. I like to have 5-6 full res backups rotated amongst drivers and servers including Time Machine.

The trick is to have two macOS user accounts with the same Apple ID [2]. One you use strictly for backup.

This is what I did starting with my Library in full res mode on my primary account. It takes about 610GB or so.

1. Copy Library to external drive.
2. Change primary Library to optimized storage [1].
3. Create new user account called iCloudBackup.
4. Log into iCloudBackup using same Apple ID as primary account. Turn off every iCloud feature.
5. Open full-resolution Library on external drive and make it the system library.
6. From within Photos turn on iCloud Photos support and set to full image download.
7. Keep the iCloudBackup account active for a few days so all sync things settle out. Then shut down.

Backups now have to include the external drive of course. You need to set something up so you periodically launch the full storage account and let it sync things up.

PS. You can track sync activity using nsurlsessionid network traffic in activity monitor.

PPS. You think -- but now I'll be using a lot of extra backup storage. Because I'll backup my full res library in addition to the primary library which is still full res until storage pressure causes a purge. Ahh, but APFS is weirder than you know. I believe that if both backups are to the same APFS drive the true storage increase will be under 100GB (due to APFS deduplication).

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[1] In Optimized Storage my library is 30GB or so with optimizes storage enabled. In reality it's about 610 but that will shrink over time as needed. APFS storage is weird.

[2] I forgot that was allowed -- two accounts with same Apple ID. Kudos Claude 4.5 for the plan.