unable to bootup after Yosemite install. have to keep shutting down from rear button. it does come on after a few times but then hangs at user login again. If you get in it is great. Seems faster than Mavericks but there is some sort of issue, bouts

You might want to restart holding the Command and R keys so you start on the recovery HD, then from Utilities on the menu bar choose Install Mac OS X and let it reinstall Yosemite. That does not remove any settings or content, just replaces the Mac OS X files and should replace whatever is broken.

I agree. After using CMD R I am able to install only Maverick again. So, I have the same problem but I finished just at the beginning - so, kernel panic and no able to even install Yosemite.:-(

I had a similar problem on an iMac Found that if I removed my McAfee anti-virus (which I didn't really need anyway) by booting into Safe Mode (hold shift after a restart), and running its uninstall program it all worked.

Had the similar problem booting up after installing Yosemite - wouldn't boot up fully, stalled half way during boot up. so I did the Command+R to recover my last Time Machine save. I won't be going back to Yosemite any time soon. I thought Apple were better than this.

So youre saying McAfee antivirus is the problem with rebooting Yosemite?

Yes tried reinstalling Yosemite but didn't work for me so I went back to recovery in Time Machine's last save.

Had a similar problem. When I removed McAfee as suggested by someone in this community it solved the problem and Yosemite now works.

Well, I'm saying removing McAfee solved my problem! In fact, by way of experiment, I re-installed it after I'd got everything going (risky!) - and it caused a problem again - so I removed McAfee using the uninstall and all seems fine. Don't know if it's just McAfee or other anti-virus programs (which they say you don't need with Mac OS X anyway)>

Looks like it might well be McAfee is also my problem must un-install it right away. Thanks

My friend was freaking out that her MacBook would be stuck at the 40% mark when booting up. Her applications would start freezing, and other extensions were giving her problems. After spending hours on finding a solution to this, a couple of people decided that downgrading to Mavericks would be the best bet. When I was installing Mavericks to make an install flash drive, I noticed she had McAfee installed. I remember reading posts about antivirus softwares giving problems for Yosemite.

I booted into safe mode, uninstalled McAfee, and she could finally boot-up fine and have no other problems. Thank you!

Knock on wood, this seems to have been my problem as well. I stumbled on this thread after dealing with this ridiculously-long boot times for the past several weeks.

I just reinstalled McAfee Antivirus and my Macbook Air booted up in less than 1 minute. No more hanging on the boot-up progress bar. No more hanging after I click on my user's avatar on the log-in screen. Bootup would often take 5+ minutes and sometimes never complete.

This has been SUPREMELY frustrating.

THANK YOU SO MUCH FOR POSTING YOUR RESPONSE!!! 😁