TL;DR: Yes. But disposing it won’t free much space because most of the image files are shared between the two libraries. See below for the details.
When Photos migrates an iPhoto library, the new library has hard-links to the same image files that the old library uses. So you can delete either one (but not both, obviously) without losing content, and without duplicating all the data.
You can see this from a Terminal session using the du (“disk usage”) command to view the sizes. du understands hard links and doesn’t double-count files if multiple links appear as it runs.
For a quick example of what I’m describing, I’m going to make a directory containing a copy of the system sounds. Then another that contains hard-links to those same sounds:
$ mkdir ~/tmp
$ cd tmp
$ mkdir sounds
$ cp /System/Library/Sounds/* sounds
$ mkdir linked_sounds
$ ln sounds/* linked_sounds
If you do an ls -i (to see the internal inode numbers that represent the actual file storage, you’ll see that the files in the two directories share the same inode numbers - proving that they are two directory entries referencing the same files:
$ ls -i sounds linked_sounds
linked_sounds:
84952759 Basso.aiff 84952766 Morse.aiff
84952760 Blow.aiff 84952767 Ping.aiff
84952761 Bottle.aiff 84952768 Pop.aiff
84952762 Frog.aiff 84952769 Purr.aiff
84952763 Funk.aiff 84952770 Sosumi.aiff
84952764 Glass.aiff 84952771 Submarine.aiff
84952765 Hero.aiff 84952772 Tink.aiff
sounds:
84952759 Basso.aiff 84952766 Morse.aiff
84952760 Blow.aiff 84952767 Ping.aiff
84952761 Bottle.aiff 84952768 Pop.aiff
84952762 Frog.aiff 84952769 Purr.aiff
84952763 Funk.aiff 84952770 Sosumi.aiff
84952764 Glass.aiff 84952771 Submarine.aiff
84952765 Hero.aiff 84952772 Tink.aiff
If I use the du command to view the size of each directory individually, you’ll see that each contains 4.6 MB:
$ du -sh sounds
4.6M sounds
$ du -sh linked_sounds
4.6M linked_sounds
But if I tell du to tell me the disk usage for both together, you’ll see that the second (based on the order I list them) will be 0 size, because all of its files were previously counted by the first directory:
$ du -sh sounds linked_sounds
4.6M sounds
0B linked_sounds
$ du -sh linked_sounds sounds
4.6M linked_sounds
0B sounds
You can do the same with your photo libraries, which are just directory trees containing all of the image and database files representing the library. If you run du against them individually, you’ll see a large size (what Grand Perspective reported), but if you run it against them both together, you’ll see that the second will be much smaller - the size of the files that are not shared with the first.
Here’s an example from my system. Photos Library.photoslibrary is the currently-active library. iPhoto Library.migratedphotolibrary is the original library that was used to create the Photos library. But the photos library has been used for several years since the migration (deleting many migrated pictures and importing many new ones), so they no longer have identical content.
Individually, the Photos library is 77 GB and the iPhoto library is 88 GB:
$ du -sh 'Photos Library.photoslibrary'
77G Photos Library.photoslibrary
$ du -sh 'iPhoto Library.migratedphotolibrary'
88G iPhoto Library.migratedphotolibrary
But if I check the usage of both together:
$ du -sh 'Photos Library.photoslibrary' 'iPhoto Library.migratedphotolibrary'
77G Photos Library.photoslibrary
24G iPhoto Library.migratedphotolibrary
$ du -sh 'iPhoto Library.migratedphotolibrary' 'Photos Library.photoslibrary'
88G iPhoto Library.migratedphotolibrary
13G Photos Library.photoslibrary
In other words:
- The Photos library has 13 GB of data that is not present in the migrated iPhoto library
- So deleting the Photos library would only free up 13 GB of space, not 77 GB.
- The migrated iPhoto library has 24 GB of data that is not present in the Photos library
- So deleting the iPhoto library will only free 24 GB of space, not 88 GB.
- There is 64 GB of data that is shared by both libraries
- The total disk usage for both libraries is 101 GB (not the 165 GB you’d get by adding together the size of the libraries measured individually).
It also means that if you drag/drop these libraries to new storage volumes (which will probably not preserve the hard links), the result will consume 64 GB more than they did in their original locations. Unless you copy them together using a utility that recognizes the hard-links and can preserve them on the destination device. I don’t know if any common Mac utilities (Finder? CCC?) will do this.
Unfortunately, accurately representing this sort of thing can be really tricky, so I’m not surprised that Grand Perspective (or any other tool, I assume) double-counts shared storage.
I think it would be really hard to present this data in a way that shows how two directories/packages are large individually but share a lot of storage with each other.