Sequoia: 400 GB free storage but not really: How to force Photos to downsize using large encrypted disk images

I read Eclectic Light so I know about macOS APFS free space chaos. Recently macOS Photos quietly grew to consume most of my boot drive leaving only 30GB free. I switched to optimized storage while preserving a full local library on an external drive.

So Finder showed 400GB free --- but it's "purgeable free". So not really. One of my sparse disk image ran out of space (causing cryptic error messages) because it couldn't grow even 10GB.

In theory the OS should be shrinking the Photos Library as needed. But if it's doing that it's doing it slowly. An ai claims iOS Photos is much better at shrinking than macOS Photos; macOS Photos optimizes only under severe pressure (10GB free?!) and then only slowly.

I created sets of 20GB images followed by a shutdown and restart. Over an hour or so of this I was able to get 90GB truly free. That's enough free I can just chip away at the rest by creating then eventually deleting larger disk images (regular, NOT sparse). Just make sure you don't back them up accidentally - wastes space. A full shutdown appears to be needed as part of the routine (magical incantation).

(The other approach is to nuke local photos and let it rebuild optimized from iCloud.)

PS. There are so many bugs in modern Apple software I only post workarounds for the big ones. I will be glad to see Tim Cook retire.

Update 11/25/26

Some further refinements

• Create a folder to hold the disk image and in Time Machine set as no backup
• Use Disk Utility to see the real free space
• Create an encrypted (reduce compression possibility) disk image about 8-10 GB less than true free space. (If you make it too large Photos can't iCloud sync, about 8-10GB seems to work)
• Let things sit for a few hours.
• Shut down and restart
• Delete image and empty trash
• Shut down and restart and check Disk Utility for true free space
• Repeat every few days until Photos is beaten back
• Pray Tim Cook has a happy retirement

Once Photos starts optimizing it seems to work in bursts, sometimes freeing up 5-10GB in a burst of activity. Photos doesn't need to be running; folklore says it is faster if it's not running but I have not verified that. The full shutdown does seem to be necessary to see true recovered space. You can run Photos to verify that it's doing iCloud sync.

It's funny to see the different size estimates in Finder vs Storage (preferences) vs Disk Utility. In Storage Photos uses 35GB (aspirational!). In Finder it used 512GB originally, now down to 467GB and shrinking. Disk Utility showed about 15 GB free space, Storage says about 500 GB free, Finder Info says 500GB purgeable.