External drives · Media libraries and starting up from a drive
How do I open a Photos library that's on an external drive?
Short answer: Quit Photos and hold Option while opening it, then pick the library on the external drive (or Other Library). Double-clicking the library in Finder works too. Keep the drive connected, and set it as the System Photo Library to use iCloud Photos with it.
Quit Photos, then hold the Option key while opening it. Photos shows a list of libraries; choose the one on your external drive, or click Other Library to find it. You can also double-click the library file in Finder.
The drive must stay connected while Photos uses it. If you want iCloud Photos to sync that library, set it as the System Photo Library in Photos settings — see moving a Photos library to an external drive.