With iCloud Photos enabled, it syncs the photos across all your Apple devices. When you turn on iCloud Photos on a Mac, all the local images in the Photos app will upload to iCloud, and the original images remain unchanged on your Mac, you will also find the photos from your other iCloud Photos-enabled devices appear in your Photos app but in low resolution, you can also download the original photos with the download button.
If you have the Optimize Mac Storage turned on, it may store some older items in iCloud when the disk space is insufficient, then, it needs to first download these photos from iCloud before you can move the iCloud Photos library to an external hard drive.
If you fail to move the iCloud photos library to an external HDD, you can troubleshoot the issue from these aspects:
Ensure there is enough space on the external hard drive.
Disable Optimize Mac Storage.
Isolate network connection issue.
