After upgrading your Mac to OS X Yosemite or later, the new Photos app gets installed automatically.
On first launch, Photos will auto-upgrade your default iPhoto library to its new library format. People will multiple iPhoto libraries must manually convert them to the new format by holding the Option (⌥) key when clicking the Photos icon in the Dock.
Upon completing migration, you’ll have two sets of photo libraries on your machine: the original iPhoto libraries and their Photos counterpart.
In order to free up a significant amount of storage space on your Mac, you can safely delete any iPhoto library that has been migrated to the new Photos app.
Before we get to it, keep in mind that even though iPhoto and Photos libraries are compatible, any changes you make to your images in Photos post-migration won’t appear in iPhoto, and vice versa.
How to remove your old iPhoto library
1) Click the desktop and in the Finder menu choose Go → Home.
2) A new Finder window will open. Navigate to your user account’s Pictures folder, or click Pictures in the lefthand column in the Finder window.
3) Locate an old iPhoto library you wish to remove, right-click the file and select Move to Trash in a contextual menu.
Tip: iPhoto libraries have the “.photolibrary” extension, while libraries in the new format tailored to the Photos app have the “.photoslibrary” extension.
4) As a final step, trash iPhoto (it’s in your Applications folder) if you don’t envision using the app going forward.
How to migrate photo libraries from iPhoto to Photos
If you bought a new Mac with OS X Yosemite or later pre-installed, or performed a clean OS X install on your machine, you won’t see the iPhoto app. iPhoto will, however, continue to live alongside Photos only after upgrading from a prior OS X version to OS X Yosemite or later.
To manually migrate an iPhoto library to Photos, drag the library file inside your Pictures folder and drop it on the Photos app’s icon in the Dock. Photos will launch, load the selected library and save it in its new format, leaving the original iPhoto library file intact.
Switching between multiple libraries in Photos
To switch between multiple image libraries on your Mac, hold down the Option (⌥) key when clicking the Photos app in the Dock. Now select Choose Library to switch to an existing Photos library; Other Library if the library you wish to switch to is not listed or you’d like to load a Photos library from an external drive or location other than the Pictures folder; or Create New to start a new Photos library from scratch.
Why you shouldn’t remove old iPhoto libraries
When you launch the Photos app for the very first time, you’ll be given the option to migrate a photo library from iPhoto or Aperture to the Photos app. After the process completes, you’ll have two photo libraries inside your account’s Pictures folder—one is the original iPhoto library and the other is your migrated library in Photos.
However, the two libraries don’t take up twice the space on your hard drive although the Finder reports them as two separate bundles. Thanks to Apple’s smart engineering, the Photos app creates a new library structure but doesn’t duplicate your images when migrating iPhoto libraries to Photos.
Instead, Photos saves disk space by creating links to the original and preview versions of any images present in your old iPhoto library. In other words, your Photos libraries and their iPhoto counterparts are not gobbling up huge amounts of disk space together.
“When Finder reports the file size of your Photos library, it includes all your originals and previews,” explains Apple. “It may look like your remaining iPhoto or Aperture library is taking up twice the space on your hard drive, but it isn’t—your images exist only in one location, even though you may have more than one photo library.”
You shouldn’t really feel tempted to delete your original iPhoto or Aperture libraries after migrating them to Photos, Apple argues, because “the migrated library takes little additional space.” To reiterate, all of your original images and previews exist only in one library file on your Mac, regardless of whether or not you have seemingly duplicate Photos/iPhoto libraries.
And due to the nature of hard-linking between Photos and iPhoto, deleting your old iPhoto library won’t remove migrated images in your Photos library.
To learn more about hard-linking between iPhoto and Photos, check out an informative backgrounder published on Six Colors.
I, for one, prefer not having duplicate libraries because hard links that point to the original images reduce the reported and usable free storage space like real files do.
If you’re going to trash your old iPhoto library, you’ll be safe as long as you migrate it to Photos beforehand. After deleting the iPhoto library file, I suggest you make sure that all of its images are still present in the Photos app. It may also be a smart idea to perform a backup of your Mac using Time Machine prior to deleting any photo libraries.
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