This is routine message. The reason is, that only one of your Photos Library can be the system Photo Library. And only the System photo Library can sync with iCloud Photos. That is why iCloud Photos will be disabled for your current Photos Library, when you make a different library your System Photos Library.
Before you make the moved library on your external volume your System Photo Library, you may want to check two things:
- Make sure, that your current library on your system volume has finished uploading all photos and videos. The status bar below"All Photos" should no longer be showing "Uploading ... " but "Updated just now" or just showing the number of photos and videos.
- Make sure, your external volume with the moved library is compatible with Photos as described here: Move your Photos library to save space on your Mac - Apple Support . The volume needs a wired connection (USB, or Thunderbolt, not a network share), it must not have been used for Time Machine backups, the file system format should be MacOS Extended (Journaled) or APFS. As a test, try to set the "Ignore Ownership" flag. If it is not possible, the external volume is not suitable.
Then you can safely open the library on the external volume and enable "Use as System Photo Library" and iCloud for the moved library.
- Quit Photos, if it is running.
- Double-click the library on the external volume to open it in Photos.
- Open "Photos > Preferences > General. The "Library location" should now be showing your external volume.
- Click "Use as System Photos Library". Proceed, when the warning appears. Photos will remove the "Optimised Versions" from your library, that are only in iCloud. That cannot be avoided.
- Open "Photos > Preferences > iCloud". Enable "iCloud Photos" and "Shared Albums".
Photos should now starts to upload all photos again to iCloud and then start downloading the missing photos from iCloud. If you want to avoid the new, lengthy upload to iCloud, keep your copied library as a backup, but create a new, empty lPhotos Library on your external volume and make the empty library your System Photos Library and then the iCloud Photos Library. It will be much faster and less network traffic, because you avoid the new upload to iCloud.
