Photos Library Won't Open

How is the external drive formatted and connected to your Mac? It must be either APFS or OS X Extended (journaled) and both preferable with Ownership set to be ignored. And it must be connected directly to your Mac via cable.

What model and year Mac do you have?

As a first try and as a test launch Photos with the Option key held down and try to create a new, test library.  Import some photos and test to see if it wo4ks as expected. Does it? This tells us if the problem is limited to your current library or is more wide spread. Then close that library and try opening your current library again.

Are you running a "managed" or "referenced" library"?

Do you have iCloud Library enabled and, if so, is "Optimize Mac Storage" enabled?

How big is you library?

If the above works but not your current library try opening it with the Option+Command keys held down to repair the library. NOTE: if you're running an iCloud Library this will trigger a complete upload again. So you may not want to try this right away.

How old is the external drive? An old drive can start to fail, independent of software updates.

You may want to check your external drive with Disk Utility, if there are any issues.

Something must have changed. Have you accessed the library on the external volume from a different mac? Or run any third-party software to remove duplicates?

Does your external drive meet all requirements, described here? Move your Photos library to save space on your Mac - Apple Support. The drive must not have been used fro Time Machine backups, the file system format needs to be MacOS Extended (Journaled) or APFS, the drive should be directly plugged in, not accessed over the local network. If you cannot set the "Ignore Ownership on this volume" flag, use a different drive.

Also check, if you are having more than one Photos Library. Photos may be trying to open a different library.

You can select your Photos Library directly in the Finder and try to open it by double clicking it. Can you open the library directly, if you double-click it?

I doubt that upgrading will fix the problem. Photos 8 on macOS 13 Ventura is even more restrictive, if the external drive is not fully compatible.

Have you checked, if the external drive is compatible? The most important thing to check ist the file system format. You can see the format, if you select the drive in the Finder and use the keyboard shortcut ⌘i to open the Info and open the "General" brick: Is it APFS or MacOS Extended (Journaled) as it should be?

And please confirm, that this external drive has not been used for Time Machine backups.

Also confirm, that you could set the option "Ignore ownership on this volume" at the bottom of the Info, as described here: Move your Photos library to save space on your Mac - Apple Support

As we cannot see your Mac, you have to look for us and tell us the relevant information.

<<Are you running a "managed" or "referenced" library"?

I don't know. How can I tell?

<Do you have iCloud Library enabled and, if so, is "Optimize Mac Storage" enabled?

Not using it for photos. Just music and files

What do you mean by that? You can't use the iCloud Library for music and files. Can you eWhere is your library located? If on an external HD how is that drive formatted and connected to your Mac? air.

Was the library ever opened with a newer system and Photos app? If it was that would give you that warning. Did you upgrade to a newer system and then downgraded back to your original system? Or open the library from another Mac that was at a newer system?

What system are you currently running on your Mac?

Did the library ever get opened by another Mac running a newer system?