As you know I’m trying to make space on my MacBook by Deleting a bunch of redundant images from my Photos Library. 

As a backup, I saved the Library on an external hard drive ... just in case I messed up images I wanted to keep.

I went to confirm the library was copied over properly but when I clicked on it in my external hard drive, this popped up (see below). 

What should I do? 



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This message is trying to tell you that your library contains full res versions as well as "optimized" or thumbnail versions of pictures. Therefore if you want to copy the entire library, you are not really copying all the pictures in their original file size. Since iCloud works with Photos in sync, the only way to actually drag over an entire full res version of one's library, they would have to download all full res versions of those pics from iCloud first. Some of those items were only partially downloaded and the message is trying to say that they will leave the full res versions alone in the cloud but get rid of the partial files it has already download. The good news here is that regardless of what you do with this message, you shouldn't lose any pictures as Photos would have already put them in the cloud.

I would need more info in order to answer the question more thoroughly.