If the Photos app complains that there isn't enough disk space on your startup disk to convert your iPhotos Library to a Photos Library, you may find this answer useful.
- Quit iPhoto.
- Quit Photos.
- Back up.
- Make a Work folder on an external disk that has lots of space.
- Drag-copy iPhoto Library from your Pictures folder to the Work folder.
- Open the Work folder copy of the iPhoto Library in the Photos app by dragging it to the Photos app.
The Photos app makes a new "Photos Library" in the Work folder. Only the Photos app will be able to open this new Photos Library, and only the iPhoto app will be able to open the iPhoto Library in the Work folder (because the Photos app changes its filename extension when it's done converting).
- Drag the iPhoto Library from your Pictures folder to the trash.
- Drag only the new Photos Library from the Work folder to your Pictures folder.
- Delete the Work folder from the external disk when you're sure everything went according to plan.
If you're going to do anything different from steps 8 and 9, here's something you need to know. The photo files in new Photos library are not copies of the photo files from (the Work folder copy of) the iPhotos Library. They are "hard links" to them. This means that for example the IMG_1234.JPG in the Photos Library and the corresponding IMG_1234.JPG file in the iPhoto Library are just two different references to the same blob of image data on the disk. This fact has two important ramifications:
Whenever you're ready to delete the iPhotos Library folder,
- Doing so will have no effect on your new Photos Library.
- You won't get much freed disk space (because the photo files remaining in your new Photos Library will still be holding onto the disk space they shared with the photo files you're deleting as part of the iPhoto Library).
If you want to still be able to open your old iPhotos Library in the iPhotos app, you have two choices:
- Don't delete the iPhotos Library from the Work folder. You can still open it in iPhoto by drag-opening it, and from then on the iPhoto app will remember that this is the iPhoto Library it should open. In other words change Step 9 to delete only the Photos Library. from the Work folder.
- From the Work folder, drag both the new Photos Library and the iPhotos Library to your Pictures folder, together, in the same drag. Dragging them both in the same drag will preserve the hard links between the image files in the two libraries (and thus save a lot of disk space). In other words, change step 8 so you copy both libraries at once; otherwise, you will not have the option to achieve the same effect later.*
* As of probably 10.13, you will be able to migrate your startup disk to the new APFS disk layout. With APFS in place, you will be able to copy the iPhoto Library to your Pictures folder later without wasting disk space. This is because APFS will automatically figure out which image files are the same between the two libraries and will quietly share disk space between them similar to the way hard links do.