Q: Yosemite Upgrade - iMac mid-2011 - Stuck on white progress bar screen at about 50%

Hi All really hope someone can help, Bought a new to me 2011 Macbook Pro( think i am the 3rd owner), so put myself as administrator etc before doing yosomite upgrade overnight, awoke to find it had got as far as 50% install and stopped as re other posts. So i have followed the post re Command r and turn off wifi, and restart  it then successfully started in yosomite, but with a completely different addministrator with password needed!!   so i can only assume that it has gone back to the original owner as its not the lady i purchased it from, nor me as administrator. How the heck can i resolve this?  In windows terms i would say its gone back to a previous restore point but with the new OP installed, Hoping someone can help Please

Now sorted

What was the solution to your problem?  Others may benefit from what you ended up learning.

Glad it got fixed!

Excellent suggestion!  My MBP late 2011 also got stuck at 50%.  Because at some point it started backing up judging by the noise my attached USB external drive was making i figured that the machine was actually up and running and only the display was frozen.  Most likely having to do with mismatched video driver / system caches or PRAM settings.  I managed to fix it without actually force shutting down.  I was lucky that its WiFi was already connected and  remote login (sshd) had been enabled (system preferences / sharing).  I logged in from my ipad using my favorite ssh/terminal app, and issued the following three commands:

sudo rm -rf /Volumes/Macintosh\ HD/private/var/db/BootCache*

sudo rm -rf /Volumes/Macintosh\ HD/Library/Caches/com.apple*

sudo shutdown -h now

This cleaned the system caches and then triggered a clean shutdown giving all running apps a chance to gracefully quit.  With the machine off, I disconnected the backup drive.  I then reset the PRAM by holding Command-Option-R with the left hand, pushed the power button and immediately hit the P key (while still holding Cmd-Opt-R).  The computer  played the Apple chime, then restarted.

At this point it sounded like a good idea to scan the system drive for errors, so I entered single user administrative mode by holding Command-S right after the Apple chime.  At the command prompt I ran the following command as instructed on screen:

fsck -fy

This took a few minutes but there were no errors. I then restarted the macbook by entering the command:

shutdown -r now

As it went back to the grey screen with the apple logo and the progress bar, the mouse pointer showed up at about 30%, then it stumbled for 10-20 seconds at 50%, and then quickly progressed to 100% and displayed the login screen.

All works great now!  Thanks!

I had the same problem and this fixed it. Thank you SATopWater!

Had the same issue 2011 iMac stopped at 50% for a couple hours. 

SATopWater's suggestion resolved it. 

Thanks for the help. It helped me to solve my problem

Can't thank you enough!  I sat here all day waiting for it to reboot and lost hours of work time. I read your post, tried it, it worked like a charm, and I'm back on track. Thanks again!!

I followed this "non advice" of yours step by step last night after having been stuck mid point on the Yosemite install restart for 5 hours.  All I can say is, it worked and saved me untold time and headaches of subsequent recovery efforts.  I cancelled the call I had in to Apple Support.  Thanks!!! To say the least.

This worked for me. I think it's ridiculous that Apple rolled out an OS upgrade that stalls in the middle of installing. I even waited half a year to install the upgrade and the problem still exists.

Thanks, SATopWater! Apple ought to reward you for this.

YOU SOLVED IT!

IT WORKED FOR ME TOO; I WAS  SO SCARED TO DO SO, BUT CAREFULLY FOLLOWING YOUR STEPS HELPED. THANKS SO MUCH FOR SHARING YOUR TIP GENEROUSLY; SAVED ME A CAB FARE TO THE APPLE STORE.

@ToleXElijah

Thank you for these easy to follow instructions.  This just happened to me and following your advice everything went through smoothly.   What I wonder is why this is still a problem with my late upgrade.  One would think this would have been resolved by now!

Thanks so much!

What helped for me was to start it up in safemode. Then I could back up all my files before something goes wrong

Excellent, worked for me too!

So I've been having the same issue as everyone here, stuck at 50% on Yosemite install. I left it on for 24 hours and nothing happened. It ended up taking 2 of the solutions mentioned here for things to work for me.

First I did satopwater's suggestion to reboot into recovery mode, turn off wifi, then restarted. The problem was that I hit the same 50% issue. So I kept reading and found KeWeL Image and Mac the Ducks posts saying to clear the NVRAM with (command + option + p + r). I read through the apple support page on NVRAM which is this https://support.apple.com/en-us/HT204063, restarted the app and then pressed the recommended buttons. The MacBook restarted and then the progress bar continued all the way to the right. To say I was ecstatic would be an understatement.

Thank you guys for providing for a solution to this issue.