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

I am having the same issue. I started my update last night and it's been stuck at about halfway for 12 hours.

I think I've found some explanation in this thread

Upgrade taking excessive time (15 hours and counting)

Do you have something like Homebrew, Macports, or TeXLive installed where you have lots of installed files outside of /Users that weren't installed by an Apple installer?

No, I do not have any of those apps. It's just been sitting on that half way point forever. I have rebooted and it just goes back to that spot.

Hmm. If you don't have a lot of stuff outside of /Users and /Applications that have been installed through non-Apple mechanisms then I'm at a lose, but that has been the best guess so far as the cause of the excessive delays.

I should say that after 16 hours the iMac I described in that thread did finish. So perhaps you should see what happens if you just wait it out.

also having this problem on a 2011 era macbook air. left running over night and still at 50%. Multiple reboots still stuck at 50%

I have a MacBook Pro from the same time and same issue. It's been 15 hours now and nothing. I have a call into Apple so hopefully we'll see what's up.

Let us know what they have to say please.

I have the same 2011 Mac Book Pro and the install boot scree hung at exactly at 50%. I performed the same exact procedures and it came up immediately! Funny thing is, I had already disabled WiFi before the upgrade anyway, however it was enabled after the first boot. Thanks for the tip!

This did the trick! Thank you. The Apple Care guys said my only option was a wipe clean. So glad I had your tip.

This worked for my MBP 17" mid-2010 after 10 hours of minor panic watching progress bar stuck at 50%:

Reboot and hold SHIFT key to go into Safe Mode and keep holding that SHIFT key.

*Patience is the key to the Shift key*

Keep the Shift key down.

Do not let go of Shift key until you go to Italy when the computer does its Venetian blinds thing and goes into Safe Mode.

Check whether you have any login items. Do this via via Sys Preferences > Users & Groups

It is going to be slow. You are in SafeMode.

Remove any login apps.

Run any updates for Yosemite (e.g. Pages, Numbers, Keynote) within Safe Mode.

Reboot.

Remember, YMMV

Tried on mbp mid 2009 and fixed the problem

I do the following

- Switch iMac on as usual and allow 5-10 mins for something to work.

- Press and hold the power on/off button at rear for 10 seconds and then power back on.

- You should now be up and running.

I am not sure what else to suggest or whether this is a good idea. It works for me, despite having to effectively switch it on twice its working.

Hoping some kind of update will be offered in the very near future, appears to be a great deal of these shenanigans.

We are talking about the rebooted progress bar...the gray screen with the apple and bar underneath, correct? Mine has been stuck at 50% for 3 hours and I'm trying to figure out my next step...

SATopWater - totally worked! Thank you!