How can I see where Photos.app thinks the System Photo Library is?

Since I upgraded to Big Sur, Photos.app is saying it cannot find the System Photo Library. Apparently, Photos.app is looking for it in some place where it is not.

Not necessarily. It could mean, that the external volume with your libraries is having permission issues and Photos cannot look there for your Photos Library.

Has your external volume been used for Time Machine backups? Then Photos will not look there for the library. As a test, try to set the "Ignore ownership on this volume" on your external volume as described here: https://support.apple.com/en-us/HT201517 . If it is not possible, something is wrong with the external volume.

Photos may also signal that the System photo Library is missing, if you did not connect the external volume, before you logged into your user account. Make sure, the external volume is always plugged in, before you start up your Mac and sign into your account. The system photos library needs to be available for the background processes, as Lon as you are signed into your account, even if you are not working with Photos.

If you open each one in turn by double-clicking it, you could identify the System Library from the Preferences | General tab. The System Library will be greyed out while the others have a clickable button 'Use as System Photo Library'.

Stephen

Hold down the option key while you open photos.

The system library is identified by (System Photo Library) after the name.

The path to each library is shown under the list when you select it...

In any case - open the library you *want* as your system library. Go to photos settings, general tab and click "use as system photo library"

If that button is greyed out then it already is the system library - and then maybe it is mount time problems as mentioned by Leonie.