Hello first time poster…
So as I read your post I shudder to think that you want to delete your photos from your Macbook Air.
Of course I understand that is your whole purpose of buying an external hard drive and that is to free up the space that is used on the Macbook Air. However you always should have at least two copies of your files, unless you have a method of retrieving copies like Mp3s’ that could be retrieved from itunes.
Storing two copies on the same hard drive doesn’t count.
Yes, that does mean buying two 3TB WD my cloud drives or one 3TB WD my Cloud and one 3TB WD USB drive for backup.
Alternatively, leave the original on the Macbook Air and use the Cloud as your backup.
Another solution would be to take advantage of the NEW Unlimited Google Photos that lets you upload unlimited photos to the Cloud as your secondary back up storage. Notice that I said secondary back up meaning don’t go returning your WD Cloud drive and just use Google Photos as your primary storage. I would never trust Google as a primary storage medium as they have the right to take away the service at any time thus losing all your uploaded photos, they could delete your photos anytime, resize or compress your photos thus changing the quality of your photos, they could even possibly use your photos for advertising although unlikely since everyone would cry foul, but at least you have a secondary copy of your photos.
I understand that many people are doing exactly what you are doing and that is moving your photos and other files from the main computer over to the external drive and had had no problems.
However once you have lost all your files “just once” due to hardware failure or other reasons, you will begin to understand that keeping one copy is never enough.