Photo cannot find system library
i have a problem. I have my photo library on an external drive. I try to open it (using the option key), but I get the message that Photo can't find the system library. I cannot change the system library either from Preferences in the menu bar. The library file seems ok in itself. But it cannot be opened by Photo and I have no idea how to open it now. Anyone who can help?
MacBook Air 13", macOS 10.15