Consolidating Iphoto Libraries

Mac users coming from iPhoto may wish to move an iPhoto Library to the new Photos app. While importing is an option when first setting up the Photos app in OS X, many users many have skipped the initial setup screens and missed that opportunity to import pictures and images into Photos from apps like Aperture and iPhoto. How to Move iPhoto Library to a New Location/Computer iPhoto is a part of iLife software application suite, which not only helps you organize and edit your photos, but also lets you get more from these photos by making photo books, greeting cards and slideshows. IPhoto will generates a iPhoto Library folder or package that contains imported photos and any albums you've added using iPhoto.

On your new Mac you can open an old iPhoto Library into Photos by holding down the Option ⌥ Key as you start Photo's. You can the choose the iPhoto Library and you should be able to open it. Photos will then convert it into the new Photo's format, so I advise you make a backup copy before you start. Step 5: If the Photos Library is empty as confirmed in Step 3, delete the Photos Library database from /Pictures. We do this to avoid having multiple libraries for the Photos app, and to prevent weird names like “ Photo Library 2 ”. Source: Moving iPhoto library to Photos app. The new Photos app will include some of the features contained within iPhoto and Aperture, but also offers the big advantage of syncing with iCloud Photo Library, which will sync every photo you.

I have several iPhoto Libraries. In theory they can be consolidated with iPhoto Library Manager (which I've registered) or one of several potentially risky techniques, but my tests a few years ago did not go well.Apple's answer to this problem is 'buy Aperture'. Since I'm testing Aperture 2.0 prior to purchase I tried merging two libraries 'by reference':

Consolidating Iphoto Libraries Mac

Macworld | From iPhoto to Aperture
... When importing pictures, Aperture can copy images into its library, thus creating duplicates of each picture, or it can reference, or point to, the existing files in your iPhoto Library folder. Although you’ll save disk space by referencing your iPhoto files, you’ll lose out on a key advantage of using Aperture: its Vault feature won’t back up referenced files (see “Why Move to Aperture?”).
If you’d like to try out Aperture but aren’t ready to import your images into the program, consider using referenced files as a trial run. When you import images from iPhoto, go to the Store Files pop-up menu in Aperture’s Import pane and select In Their Current Location. If you later decide that you want to use Aperture as your main photo-management tool, you can import the original files from iPhoto by selecting File: Consolidate Masters....
I imported two iPhoto Libraries into a single Aperture Library this way. Each became a project.
The first thing I checked was whether the image counts matched up. There were 2071 + 1772 = 3843 images in my two imported Libraries; but this is the count of 'current' images. It doesn't count the originals of any edited images.
Aperture imports an original and 'current' image as a stack, so each image is counted. I typed opt-; to collapse all stacks and, happily, my image count then matched the iPhoto total.
So that was nice! Those Libraries didn't have Videos however, and we know Aperture can't handle video files. So we'll see how that goes.
The images keep their titles, comments, ratings and keywords. Smart albums (annoyingly) become 'dumb' albums. Much worse -- comments on Rolls, Albums (Collections) and Events are completely lost. Aperture can't handle metadata at that level.
Slideshows and photo books, as well as videos, can't make the move either.
How to consolidate multiple iphoto librariesLibraries
That's a lot of lost metadata...
Consolidate iphoto libraries

How To Consolidate Iphoto Libraries

How to consolidate multiple iphoto libraries

Merge Iphoto Libraries App

Consolidating Iphoto Libraries

How To Consolidate Photo Libraries On Mac

Update 2/17/2008: I added a third iPhoto Library of 8,000 items, again by reference. This killed Aperture. I got an endless SPOD (spinning pizza of death) when I tried to browse the complete image collection. I had to kill Aperture.