I've got the Photos library on an external SSD. The SSD is 512 GB with a real capacity of about 500 GB according to Finder. The Photo library is around 420 GB according to iCloud.
When there was about 15 GB free on the SSD, the library stopped syncing, complaining about too little local storage to sync. I saw some hints on disabling and re-enabling iCloud sync, but that did not help, now I am not able to re-enable sync. The error message I get is "Your full-resolution photos and videos stored in iCloud will not fit on this Mac."
It seems Photos refuses to fill my drive above some limit, which would make sense for the system disk, but this is an external disk specific to this purpose. Is there any way to tweak this so I can use all of the disk space I got?