Move your Photos library to save space on your Mac

Make sure that your external storage device, such as a USB drive or Thunderbolt drive, is formatted for Mac: Either APFS format or Mac OS Extended (Journalled) format.

You can't store your library on a storage device used for Time Machine backups. And to avoid possible data loss, don't store your library on a removable storage device, such as an SD card or USB flash drive, or on a device shared over your network or the internet, including over a cloud-based storage service.

1. Quit Photos.
2. In the Finder, open the folder that contains your Photos library. By default, the Photos library will be stored in the Pictures folder of your home folder. You can open your home folder by choosing Go > Home from the Finder menu bar.
3. Drag the Photos library to a location on your external storage device. Find out what to do if you've received an error.
4. After you've finished moving it, double-click the Photos library in its new location to open it.
5. If you're using iCloud Photos, designate this library as the System Photo Library.

If you want to delete the original Photos library to save storage space on your Mac, you can drag it from its original location (the Pictures folder of your home folder) to the Bin. Then empty the Bin.

Always make sure that your storage device is turned on and available before opening Photos. If Photos can't find the device, it will stop using the Photos library that's stored there.

1. Quit Photos.
2. Press and hold the Option key while you open Photos.
3. Select the library you want to open, then click Choose Library. Photos will use this library until you've opened a different one.

1. Select your external storage device in the Finder, then choose File > Get Info. 
2. If the information under Sharing & Permissions isn't visible, click the triangle  to see more.
   
3. Make sure that the "Ignore ownership on this volume" box is ticked. To change it, you may first have to click the lock button  to unlock the setting. If the storage device hasn't been formatted correctly for Mac, or was used for Time Machine backups but hasn't been erased, this tickbox won't be present or won't be selectable after unlocking. In that case, erase the storage device first.

If you're experiencing some other permissions issue with your library, you may be able to resolve the issue with the Photos library repair tool.