How to move Photos Library and associated iPhotos Library to another drive without breaking links?
So, no definitive answer.

I had copied my iPhotos, Aperture and Photos libraries to the new drive using Finder, but I was worried I was using unnecessary space. I did a new copy using rsync as follows:

rsync -azH --progress Aperture\ Library.migratedaplibrary iPhoto\ Library.migratedphotolibrary Photos\ Library\ \(Aperture\).photoslibrary Photos\ Library\ \(iPhoto\).photoslibrary /Volumes/Secondary\ SSHD/Users/me/Pictures


Now I'm sure I'm using too much space. :-)

This command should preserve hard links from the source in the destination, but as @Tetsujin commented on the question APFS doesn't support hard links and I don't think it would use copy on write with files in different media (what would happen if I ejected the media?).