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I have the same problem.

I updated a secondary iMac today to 10.10.3 using the app store Update pane (an iMac on which my main iPhoto library does NOT live, just to be safe...)

after updating, I see the new Photos app, but cannot launch the old iPhoto app (v9.6) - I see the same error message as the OP when I try to launch it.

also, I notice that the old iPhoto application icon now has a white circle with diagonal slash - the kind of icon we saw back when PowerPC apps stopped being supported by the OS

so, contrary to previous articles that we'd be able to continue using the old iPhoto app after the new Photos app came out, this doesn't seem to hold true if updated to 10.10.3

p.s. while iPhoto no longer works, I have Aperture 3.6 and that still launches ok under 10.10.3

Based on advice I got in the Apple support forums, delete iPhoto and then redownload it from the Purchases tab in the App Store. That properly downloaded iPhoto 9.6.1 for me. My guess is that Aperture would behave the same way.
 
Based on advice I got in the Apple support forums, delete iPhoto and then redownload it from the Purchases tab in the App Store. That properly downloaded iPhoto 9.6.1 for me. My guess is that Aperture would behave the same way.

Thank You! That was an easy fix, I should have thought of it lol. It's weird that it wouldn't update 9.6 to 9.6.1 though.
 
Can anyone view image size of 7680 × 4320 in Preview in their iMAC 5K running latest version 10.10.3? I can't view an image of that size, it opens but disappear and only grey background left.

Here is the picture for testing: https://www.dropbox.com/s/rqofr2xuba...d7680.jpg?dl=0

It just on my iMAC 5K that I can't view it in Preview. My Macbook Retina can open just fine. I am wondering if it is software bug related to iMAC 5K or is it my computer.
 
I encountered a similar problem to that of the OP, while using the 10.10.3 public beta releases.

That is -- once I installed the 10.10.3 beta (with Photos), the existing version of iPhoto that I was using would no longer open.

How I solved it (may not be compatible with the user standards at forums.macrumors.com, so moderators remove this if it is your duty to do so):

Build yourself a url:
Start with the usual "http://"
then add behind it "swcdn.apple.com/content/downloads/36/61/031-20637/"
then add behind that "jpzsx8fqudqkj8yw7l7bktn8s5h0p4kzom/OSXUpd10.10.3.pkg"

Combine all three elements of the above into a single URL (no space characters), and paste that into your browser.

You will receive "a package", that -- when you run it -- may update your existing older version of iPhoto into one that co-exists with Photos.

No promises.
But it seemed to work for me...

So I noticed that this update was installed at time of my new full install of 10.10.3. My old iPhoto is still in the app folder with a circle and slash through it. I can't get a newer version to from Update through Purchased app although it shows there. This pkg I downloaded from your link information is the 10.10.3 update that was installed with Yosemite I think. Would reapplying this fix the iPhoto showing as disabled? The iPhoto I have on my machine is 9.5.1 was never offered and update to 9.6.1. Thanks for clarifying.
 
Huh?

And quite frankly, it's replacement is so much of a step back that even after trying it several times, I still go back to iPhoto. No external editing support, no maps view for ALL photos, and the horrible UI are just a few of the reasons why I havent switched. If Apple improves it, maybe I will switch. But for now, I will stick with iPhoto. Photos is garbage, IMO.

It's been discontinued. No more updates.

Sounding a little bit like Norman and his mother.
 
Based on advice I got in the Apple support forums, delete iPhoto and then redownload it from the Purchases tab in the App Store. That properly downloaded iPhoto 9.6.1 for me. My guess is that Aperture would behave the same way.

aha, this fixed the problem. I deleted iPhoto (v9.6) and from the Purchases tab I was given v9.6.1. I can now open iPhoto and the migrated library

Interesting that iPhoto 9.6.1 wasn't automatically included when I updated to 10.10.3. I notice on my other iMac still on 10.10.2 it also has iPhoto 9.6 and isn't offered an update to 9.6.1 in the App Store. Maybe if Apple fixed this in the offered updates, this problem could be avoided in the first place.
 
I had the same problem... I don't know why. I think its because I didn't do the iPhoto update that came out a few days before 10.0.3 came out.

I ended up fixing it after a few goes. Initially I deleted it, but the purchases tab kept saying 'Update' instead of download. It took a few restarts and fiddling to fx it.

----------

It's been discontinued. No more updates.

Sounding a little bit like Norman and his mother.

Regardless if its been discounted or not.. iPhoto is still a heap better than Photos. Photos is terrible for anything other than the most basic things. In addition to that its slower and buggier than iPhoto.
 
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