Duplicate Photo library for Photos & iPhoto

Solution 1:

I'd say stick to the iPhoto Library if that's the app you're going to continue using for now.

When you eventually swap to Photos, do the import again.

The Photos Library is greyed out if you attempt to open it from iPhoto. Any changes you make to the iPhoto Library will not be reflected in the Photos Library, meaning you'll gradually go out of sync until such time as iPhoto no longer works (OS X 10.11, who knows?) & you'll need to reimport at that time.

Photos Library Storage Space
The Photos Library is actually just a collection of hard links to the actual data on the disk & therefore is not taking up anything like the amount of space that Finder is reporting.

More on hard links from Ars Technica:

A hard link is simply a reference to some data on disk. Think of a file as a combination of a name and a pointer to some data. Deleting a file really means deleting the name portion of that duo. When there are no more names pointing to a particular piece of data disk, then that disk space can be reused."

This means that so long as one of the pointers to the data still exists, the data is still in use. Only by deleting both will the data actually be deleted.

Ref: Apple KB - Photos saves disk space by sharing images with your iPhoto or Aperture libraries

Solution 2:

As @Tetsujin pointed out, you don't have two libraries taking 50GB of space: when you import iPhoto's library to Photos, it creates something called hard links to your photos. From Jason Snell at Six Colors (emphasis mine):

Mac users are probably more familiar with the concept of soft links, also known as “symbolic links.” Mac users would recognize the idea of a soft link from the long-time Mac concept of aliases. In both of these cases, there’s something that looks like a file or folder/directory that’s actually just a reference to the real version of that file somewhere else in the filesystem.

Hard links aren’t like that. The best way to think of a hard link is that the contents of a file appear to exist in more than one location. If a file has two hard links, and you delete one, the file isn’t deleted—because it’s still linked to from another location.

That’s what the iPhoto import inside Photos does: It creates hard links to the contents of your iPhoto library inside the Photos library. If you delete your iPhoto library, the files that were hard-linked from the Photos library still exist in the Photos library and aren’t deleted. For Mac users used to the a-file-is-a-file approach of the Finder, it’s a bit of a head-scratcher.

Since iPhoto's library gets frozen when you import it into Photos (i.e. further changes don't get synced), you can safely delete it: you won't have neither more nor less space then before.

EDIT: You can also check out this answer: https://apple.stackexchange.com/a/180313/10159

Solution 3:

I already deleted my iPhoto libraries and everything looks OK. As already reported, there is no disk space gains doing that but I like to keep everything clean. Still not so comfortable with Photos but it has potential to improve.

Solution 4:

The easiest way to test this is to back up one of the library files onto an external drive, then delete it off the main drive. Then look at whether your disk usage went down or not. In my case, I like Photos, and am done with iPhoto. However, if hard links are used, I am worried the new Photos library will break when the old iPhoto library is deleted. I had 2 30GB libraries.

So, I tried it. Deleting the old iPhoto library did not save me 30GB. It did save me about 4GB. So the disk space reported is not accurate, even if you're using Omni DiskSweeper to measure it. But nothing bad happened. All my photos are intact in the new Photos software.