It has been a while since I have posted, but I figured it would be helpful to document my experience with the new Photos for OS X.

My Library Situation
Prior to the upgrade I had a few different Aperture libraries (stored in a sub directory of the Pictures directory). My primary library has a combination of internal library and externally referenced photos, some of the external are on an external drive.  A second library has all externally referenced photos, while a third library has all its photos stored in the library.  The primary library is the one that has had the photo stream turned on and syncs all of my photos from my iOS devices.

Since November I have had the iCloud Photo Library turned on for my iPad and iPhone, which has worked pretty well.  According the my iCloud.com Photos page there are 1389 photos and videos stored in my iCloud Photo Library at the current moment.

Photos for OS X Upgrade
I was very excited to come home and update my Mac to OS X 10.10.3 and to try out the new Photos app.  The upgrade took about a half an hour to download and install, but I was up and running.  I had heard a lot of talk and read a lot of reviews on the application, but looking back a lot of these seemed to focus on the standard single library that you got with your Mac use case.

I opened the Photos app and was expecting to be prompted to choose a library for importing, but instead it just asked if I wanted to create an iCloud Photo Library.  I click next and before I knew it the 1389 photos and videos stored in my iCloud Photo Library magically appeared before me. Nice!!!

But wait, what about my existing libraries?  I quickly searched online and started doing research, but I also did a little playing around with a test library containing both internal and externally referenced images.  Here are some questions I was wondering about and here are the answers.

Why wasn’t I prompted to select a library for migration? 
Since my libraries were not in the Pictures directory, but instead in a subdirectory of the Pictures directory, it just went ahead and created a new library.  Apparently if you have just one library (iPhoto or Aperture) in the folder it will automatically try to use it.

What is the deal with the ‘System Photo Library’?
You can have several Photos for OS X libraries on your system, just as you could have many different libraries for iPhoto and Aperture.  However, only one of these can be the ‘System Photo Library’.  The big difference between the ‘System Photo Library’ and the other libraries is that this is the only one that will sync up with your iCloud Photo Library.  Opening a non ‘System Photo Library’ will show you the photos in that library, but it won’t show the photos you have in the cloud and it also won’t sync your photos to the iCloud photo library.  However, you can change a library to be the ‘System Photo Library’ after the fact so don’t fret if it isn’t the one you wanted (note that you want to switch this as infrequently as possible because it does trigger a download of at preview size copies of all the photos in your iCloud Photo library.

How do I convert Aperture and iPhoto libraries over to Photos and what does that mean exactly?
If you right click on an Aperture library (MyLibrary.apLibrary) in the Finder and choose Photos from the Open With menu it will ask you if you want to start the conversion process.  Once it is complete you should see all of your photos in the Photos app.  Back in the Finder you will see that there is now a MyLibrary.photoslibrary and MyLibrary.migratedaplibrary where the original library used to be.  Going forward these are now two separate entities that can be used with their respective apps, but any changes made in one won’t appear in the other.  The first time you open a .migratedaplibrary you will be warned that any changes made to the photos will not be made in the Photos app, but if you click OK the extension will go back to .aplibrary.

What happens to referenced files when migrating to Photos for OS X?
Any files that were referenced in the Aperture library will continue to be referenced in Photos.  However, the tools for managing referenced files are very limited in Photos and essentially let you either see the referenced file in the Finder or Consolidate referenced files aka bring them into the library.  The other caveat with referenced files is that they will not be uploaded to your iCloud Photo Library.  The good news is that as soon as you “Consolidate” photos (which can be done on one or many photos at a time) they will be uploaded to the iCloud Photo Library, assuming that the library you are working on is the ‘System Photo Library’.

Can I delete the old Aperture Library after migrating?
First of all any files that were stored in the Aperture library will be stored in the new Photos library. I have read in various places that the masters in one is a link to the other so in reality there is only one copy, but I am not sure what is going on there.  In my testing I took an Aperture library with photos stored in the library, migrated it to Photos and then deleted the Aperture library without event.

My Plan
After some research and testing I think I have a plan that should work for me.  I am going to convert my primary library from Aperture to Photos.  Initially most of the library will not end up in my iCloud Photo Library because most of the files are referenced files.  This will create a single library for all of my photos going forward, with any new photos from my iOS devices or imported into my Mac automatically going into the iCloud Photo Library.  Over time I will slowly run the “Consolidate photos” operation on my referenced photos so they will be added to the library and consequently uploaded to my iCloud Photo Library.  Since I have the option turned on to Optimize Storage, space won’t become an issue and eventually all of my photos will be in the cloud.  As for my other libraries, I will probably just convert the smaller ones to Photos and the larger one leave as an Aperture Library right now because it has a ton of referenced images, which I might want the flexibility to easily move in the future which is much easier given the tools in Aperture.