Q: Photos error "an unexpected error occurred attempting to prepare your library"

Same thing has happened to me. I'm using a MacBook Aluminum (Late 2008) upgraded to 8GB of RAM and 1TB HDD running Yosemite 10.10.3. I successfully set up the photo app on an admin account that has no photos, but had the same error on my main user account.

Troubleshooting attempted so far:

1. Closed the app then tried again - did not correct the error

2. Rebooted then tried opening Photo App - did not correct the error

3. Rebooted in "Safe Mode" and repaired disk permissions - did not correct the error

I'm currently copying all my photo's to an external HDD with the intention of deleting my iPhoto library (once complete) then setting up the Photo App with no photos in my library. I'm hoping it will allow me to set it up at that point like it did with my admin account, then import all my old pics. I still have about 2 hours to go until my photos have copied over to my external HDD. I'm hoping someone will come along with a better solution. I really don't want to spend several hours/day re-organizing my photos.

I've got the same thing happening.  After 3-4 hours the new Photos app fails at 30%.  The only thing differently that I'm doing is my library is stored on a Samba share on my network.  I have a linux server where my 16GB iPhoto Library lives. Interestingly, the new "Photos" library gets created on the disk and is even larger than 16GB but the photos app crashes.  Upon relaunching, it starts the entire process over.  I've done it twice and have two of these broken "photos" libraries.

Was able to find a fix that worked for me. As you can see from my previous post Steps 1-3 did not work on their own but they may have played a part in my overall fix. Once my backup of my photos was complete I decided to try the following steps.

1. Rebooted my computer

2. Opened iPhoto by pressing and holding command and option until the rebuild photo library option appeared. I selected the "Rebuild Database" option. It says it should be used as a last resort, but it was my first choice none the less.

3. Rebooted my computer

4. Opened finder and navigated to my "Pictures" folder

5. Deleted the "Photos Library"

6. Rebooted my computer

7. Opened the Photos app.

I'm not sure if all of those reboots were necessary, but they were the steps I followed and sure enough after it was all said and done my Photos app is now working.  Good luck and hope you have the same positive results I did.

I solved my problem by copying my iPhoto Library from the network share to my local machine and doing the upgrade there.

I'm having the same issue.  After reading this and several others blogs about the update... not to mention the "optimize you computer" message I got - I've decided to continue using iPhoto. 

I truly hope apple gets back to Steve's MANTRA of, "it simply just works", in my opinion they've drifted away from it,

Option (or command, I can't remember) +clicking on the dock icon seemed to fix it for me. I just created a new library and it fired right up.

Option + click Photos app in dock let me pick the iPhoto library manually. When I selected the library, Photos started importing / converting it. Now it works!

Just to add to the former

Yes the option click while opening the app for the first time worked

This might have something to do with my library being on a separate disk

So option click and choose the existing library to import.

This works,

Dominicus

I had the same problem as OP, and your step-by-step fix helped tremendously!

I can confirm however, that while I followed your advice, absolutely NO rebooting is necessary! Thus, shortening the process for anyone else who may have this problem while updating their library to the newer Photos app.

Again, thank you.

How do you open iPhoto if you have Photos on your computer? Did you downgrade? How did you downgrade? Can I rebuild my library IN the photos app like I use to when I had problems in iPhoto?