How to free up space on your Mac by moving your Photos library to an external drive

Your Photos library holds all your photos, albums, slideshows, and print projects. If your library is large, and you want to free up storage space on your Mac, you can move your library to an external drive.
Before you start, be sure to back up your library.
You can store your library on an external storage device, such as a USB or Thunderbolt drive formatted as APFS or Mac OS Extended (Journaled). Note that you can’t move your library to a disk that’s used for Time Machine backups. To prevent data loss, Apple doesn’t recommend storing photo libraries on external storage devices like SD cards and USB flash drives, or drives that are shared on a network.
Move your Photos library to an external storage device

Quit Photos.

In the Finder, go to the external drive where you want to store your library.

In another Finder window, find your library. The default location is Users > [username] > Pictures, and it’s named Photos Library.

Drag your library to its new location on the external drive. If you see an error, select your external drive’s icon in the Finder, then choose File > Get Info. If the information under Sharing & Permissions isn’t visible, click the triangle button, then make sure the “Ignore ownership on this volume” checkbox is selected. If it’s not selected, click lock icon to unlock it, enter an administrator name and password, then select the checkbox. Note that if the volume isn’t formatted APFS or Mac OS Extended (Journaled), or has been used for Time Machine backups but hasn’t been erased, this checkbox will either not be present, or won’t be selectable after unlocking. Erase and reformat the drive for this option to be available.

After the move is finished, double-click Photos Library in its new location to open it.

If you use iCloud Photo Library, designate this library as the System Photo Library.

Delete the original library to save space on your Mac
After you open your library from its new location and make sure that it works as expected, you can delete the library from its original location.
In a Finder window, go back to your Pictures folder (or whichever folder you copied your library from) and move Photos Library to the trash. Then choose Finder > Empty Trash to delete the library and reclaim disk space.
Open another Photos library
If you have multiple libraries, here’s how to open a different one:
1. Quit Photos.
2. Press and hold the Option key while you open Photos.
3. Select the library that you want to open, then click Choose Library.
Photos uses this library until you open a different one.
If you have a permissions issue with your library, you might be able to resolve the issue by using the Photos library repair tool.
Enjoy your Mac’s extra storage space! (And back up that external drive containing your Photos library regularly.)