When you have an older iPhoto library and a newer Photos library on your Mac and you copy both libraries to an external drive, you might notice that they collectively occupy a lot more space than they do on an internal drive. Why is that? It comes down to hard links, a way of conserving storage.
When Apple released Photos for macOS in 2015, it had to have a transition plan for iPhoto libraries. iPhoto libraries could be imported and converted, but with a twist. Instead of copying all the source images, Apple used hard links, which allow a file to occupy a single place on a disk and be referenced in multiple folders as if it existed uniquely there. (You can read more about hard links in “Upgraded to Photos? Here’s what you can do with that old iPhoto library.”)
