photos.app library from High Sierra can't open in Mojave
How does one open an older Photos library from High Sierra into a Mojave desktop?
iMac 21.5″, macOS 10.12
Question marked as
hold down the option key while starting photos. Select the old library to open.
It will be upgraded for Mojave - you might want to back it up first, so you can go back to it if needed.
If it is on an external drive - is the drive formatted APFS or MacOS exteneded (journaled). If not, the library needs to be moved to a drive that is.