Updating thumbnails in iphoto
Though it may appear in the Finder that both libraries are taking up the same storage capacity, only one library — your Photos library — has the majority of the data.
Apple has a support article written about this under-the-hood magic if you want more information, but the short answer is: If you don't want to use i Photo or Aperture anymore, feel free to delete your original library. There's a setting you can uncheck to just link to your photos in their existing locations, rather than directly importing/duplicating them.
Your old photo libraries will still stay on your Mac if you decide you want to use i Photo or Aperture for doing something.
One might still want to use Aperture because it is a lot more advanced with support for plug-ins and more for professional photographers, with powerful editing tools and built-in support to use external editors.
If you were previously using i Photo or Aperture, after clicking Getting Started, it detects your i Photo and Aperture libraries. Once you select the library, it presents the option to set up i Cloud Photo Library, Apple’s i Cloud-based photo and video syncing and storage service.
This will allow you to sync all the Photos from your Mac and i OS device to each of your other devices, keeping your edits and albums in sync.
You can also check out our super in-depth Photos for OS X Ultimate Guide and i Cloud Photo Library Ultimate Guide for more information, how-tos, and walkthroughs. Your i Photo or Aperture library is compatible, and can be imported seamlessly into the Photos app.
The new Photos library references the same master images as your old library, so you don't need the space to store images twice.
Thumbnail Annihilator is included in the Duplicate Annihilator package that you can download from this site.
Here’s how to migrate your photo library to the new Photos app from i Photo or Aperture, both of which will no longer receive support for software updates going forward: After pressing Get Started in the blue box as seen above, you have two different options.
If you are brand new to photo organizing on a Mac and have never used i Photo or Aperture before, (or if you don’t want to migrate your i Photo or Aperture library to Photos), you have the option to import pictures from your digital camera or SD card, drag files directly into Photos, import pictures from the File menu or turn on i Cloud Photo Library under preferences.
Our long national i Cloud photo-syncing nightmare is over: Photos for OS X, Yosemite's official successor to i Photo, is now available with OS X 10.10.3. Your old library won't be converted, just imported, so you can still open it in Aperture or i Photo if you like (though once you import, if you do anything new with that library in Aperture, it won't carry over to the Photos version).
When you first launch Photos, it'll either auto-convert your primary library or ask you to pick which library you'd like to import.