Ran update from 10.10.2 to 10.10.3 on Mac Mini two days ago. It hung at some point as others have noted, as I wan't monitoring it. I then restarted the system and tried booting it up. It when into what appeared to be safe mode , the screen only show for a second or two. It then started the boot cycle again, & again & again in a continuous loop.
At first I thought it might be a hardware problem, so I ran the system diagnostics which showed nothing. I then went the same route others have tried, recovery mode, permission repair, but nothing changed.
Next I decided to rebuild Yosemite on an external SSD drive from the original release. It came up fine on the same mini. I then upgraded it to 10.10.3 and rebooted on the same mini. It too booted up fine. I also took the same drive over to my laptop macbook pro and it too booted up without a problem.
So I then decided to take the next step and migrate my application from the old drive to the new external drive. It took awhile but ended successfully. Then I tried booting from the new drive and I got the same problem. So my only conclusion at this point is that something in 10.10.3 upgrade clobbered something in some application. I have no clue at this point what application or utility it could be, but I have lost two days work on this issue.
Just to make sure it wasn't a hardware incompatibility, I also tried to boot up the new drive on the macbook pro and the same thing happened.
I am not upgrading any other of my computers until further notice and recommend everyone else also wait.
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