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If I have migrated an iPhoto Library to Photos (on an external disk, so independent of the computer with the apps), and then make a copy of the iPhotos Library, will I be using additional disk space?

I understand (now, but not when I did this) that the iPhoto Library and the Photos Library in effect share space because the photos referenced as being "in" both are hard linked to the same location (see Does migrating to Apple's new Photos app actually double my storage requirements? and If I've migrated from iPhoto to Photos, can I safely delete the migrated photo library?). If I make a copy of the iPhoto Library, does that make a copy of the hard links in the iPhoto Library or does it make a copy of the underlying photos as well? The copy is on the same volume, but may or may not be in the same folder (i.e. both alternatives apply).

If copying does copy the underlying photos, is there any way of telling which libraries are now sharing linked photos?

(The problem arose because I had migrated an iPhoto Library to Photos under Yosemite. I then opened the Photos Library under High Sierra, which migrated it to the new HS Photos format (something I had not anticipated), without also creating a pre-migration (Yosemite) version. The problem is that I want to use this Library on an older computer under Yosemite/El Capitan. I then tried to do my own investigations about whether the various libraries contained the same data, not being entirely sure whether there had been any changes made to the Photos Library. Additional complication: I think there were already two iPhoto Libraries: iPhoto Library.migratedlibrary and iPhoto Library copy.)

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The short answer is, of course, yes, making a copy of a library makes a real copy including new copies of the underlying photos.

A neat test to show this and to identify which libraries are linked is given by nohillside in How can I verify that my Photos and iPhoto library are hard linking to the same disk data?: compare a photo from each library using

ls -li /path/from/old/app.jpg /path/from/new/app.jpg

This will show the inode number at the beginning of each line of output. If the files are linked, they will have the same inode.

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