Downloading nearly a TB of photos from iCloud can take more than a week. You have to be patient.
You want to download the originals to the copied library on your external drive.
- Select this copy on your external drive and double click it to open it in Photos.
- In the "Photos > Preferences > General" ensure, that this library on the external drive is your System Photo Library. Click "use as System Photo Library", if it is not grayed out.
- The "Library Location" at the top of "Photos > Preferences > General" should be showing your external drive.
- Now open the "Photos > Preferences > iCloud" tab and enable "iCloud Photos" and "Download Originals" for this library.
Before Photos will start downloading the photos from iCloud to your external drive it will first have to merge your copied library on the external drive into the existing iCloud photo library.
What should now be happening:
- Photos will upload all photos from your currently open iCloud Library into iCloud. This will take several days for a large library. Do not interrupt it.
- Once this merging has finished, Photos will start downloading the missing photos from iCloud to the current iCloud Photos Library.
You can check the progress in the Status bar below the Photos View in Photos (About the status bar in Photos for macOS - Apple Support). As long as Photos is still uploading, it will not start any download and the size of the library should remain the same for a long time.
If you are sure, that all photos are in iCloud, you could skip the merging step by creating a new, empty library on your external drive instead of starting with a copy. If you make a new, empty library your System Photo Library and iCloud library, the photos from iCloud and all albums will directly download to this library, because no merging with iCloud will be necessary. You will only have the download and do not have to wait for the upload to and merging to finish.
Starting with a copy of your internal library is only necessary, if you need to save projects (books, slideshows, etc). projects will not download from iCloud, because they do not sync with iCloud.
