Q: HELP! Black screen after startup after Yosemite installation

Well, actually I had the same issue.

On a Mac Pro Late 2008 with ATI Radeon HD 4850 the installation got stuck with a black screen. I am using two Cinema HD displays on the machine.

I just for the sake of testing unplugged the one, which was connected to the mini-dvi connector. The installation went without problems. After successful install and reboot I could reconnect and use the second screen.

When I had to use the recovery partition, the crashing happened again. So my tip will be:

When installing or using recovery, just use one single screen, the one built in (for iMacs) or the one connected via DVI-Connector (Mac Pro).

The graphics driver in the install package has some obvious problems !

I too am having the exact same issue.  The black screen is entirely random.  I'd love a fix if you find one.  Cheers

Try the following, worked for me:

1. Boot while holding Command-S for "single user" mode.

2. Mount your HD:

2.1 fsck -fy
2.2 mount -uw

3. Remove the file:

3.1 cd /Library/Preferences
3.2 rm ./com.apple.loginwindow.plist

4. Reboot, typing: reboot

Extra: If mount is Read Only, try "mount -o update /" without quotes.

Reference: http://www.imore.com/black-login-screen-your-mac-heres-fix

In my case, issue was after Yosemite 10.10.1 was installed.

Resetting PRAM worked for me

Reset PRAM

Your Mac’s parameter random-access memory (PRAM) stores important information such as the type and identity of your OS X system drive, the presence of any other internal drives, the number and type of connected devices, screen resolution, and speaker volume. If your Mac isn’t acting as expected, a PRAM reset is usually the first and easiest troubleshooting step to try. You’ll also want to make sure you preform a PRAM reset after you replace your Mac’s hard drive, unless you like waiting five minutes for the system to boot while it searches in vain for the old missing disk.

To reset PRAM, shut down your Mac and find the Command, Option, P, and R keys on your keyboard. You’re going to need to power your Mac up, and then press and hold all four keys simultaneously as soon as you hear the startup chime. It’s a little tricky at first, and you may miss it on the first attempt, but just keep rebooting your Mac until you’re comfortable contorting your fingers to reach all four keys at the same time.

Keep holding the keys until your Mac reboots itself and you hear the startup chime a second time. At this point you can release the keys and your Mac should boot as normal. Note that settings such as resolution and system speaker volume will be set to defaults, so don’t be startled if your Mac’s startup chime is a bit louder on the second boot.

Same problem with iMac early 2008. After 10 - 20 min  the screen get black. Some update? Reset PRAM it's true help? looks like it depends on the video of the screen. GPU driver maybe?

After upgrading to Yosemite to same happened to my computer. A blank screen. To be sure I installed yosemite  once more and the same problem occurred.

I work on a 27 inch iMac 2010. Yosemite is not very recommendable to upgrade to. And not very Apple-like.

Your suggestion is the fix to this issue.

I work at a place with love 300 macs and all had this issue when upgrading to Yosemite from Mavericks.  The steps you provided were found on other websites, but it is the fix to correct the black screen after reboot post Yosemite upgrade.

1. Boot while holding Command-S for "single user" mode.

2. Mount your HD:

2.1 fsck -fy
2.2 mount -uw

3. Remove the file:

3.1 cd /Library/Preferences
3.2 rm ./com.apple.loginwindow.plist

4. Reboot, typing: reboot

Extra: If mount is Read Only, try "mount -o update /" without quotes.

This exactly helped me! Reseting the PAM!

The same here. Macbook pro 2010. During the boot the screen goes black and then it seems like the mac goes to sleep. Nothing helps. I installed a 3rd party SSD to create a fusion drive. Could the issue relate to this? For instance here some info http://www.larryjordan.biz/caution-ssd-drives-and-yosemite/

This fix worked for me.  I have a late 2013 Mac Pro and am using two Cinema HD displays.  I was having the black screen only on startup (not the random black screens that others have described).  I had been using a workaround by sharing the screen from another computer for the login steps (selecting identity and typing password), and had also found that disconnecting one of the displays would allow a normal startup.  But by resetting the PRAM I completely fixed the problem, and now the machine is booting normally. 

Reading some of the suggestions resolved my problem.

The update has 3 stages:

Stage 1 downloads the patch

Stage 2 restarts and shows a white screen with Apple logo in a box and starts installing showing a progress bar

Stage 3 (this is where I was stuck) "should" bring up a big OS X logo (where you can see the Yosemite picture through the X) and finish the install.

My screen went blank between stages 2 & 3 with just a cursor showing on a black background and nothing else.

After reading some of the posts (thank you all) I remoted to the Mac using another Mac (Finder, select the problem Mac & share screen). After connection was completed, I saw my troubled Mac Login screen on the remote Mac. It was waiting for me to select my username & enter my password.  I clicked on my username and put the password in using the remote Mac.  Then the iMac with problem went to stage 3 with the big X and finished the process.

I don't exactly know why it did this, but this is a new iMac with fusion drive (SSD & Spinning combo drives), and I have a username & password on the Mac.  Maybe this was a perfect storm, but doing the remote control saved me.  Thank goodness I had enabled "Share my screen" in preferences.  I hope this helps.

thank you so much. you are a god.

Saved me 4 hours of waiting at apple support.

I am having the same problem with my iMac. After I turn it on, I get the login screen, and once I log in, I get the black screen each time. I have force rebooted a couple of times, and the black screen happens every time.

I'm not as computer-savvy as some of you, but it sounds like there is a solution booting up in single-user mode.

But I don't understand the code you're offering after that. Is it something I will have to type into a prompt?

1. Boot while holding Command-S for "single user" mode.

2. Mount your HD:   (how do I do that?)

2.1 fsck -fy   (what is this? something I type on the screen?)
2.2 mount -uw  (same question as above)

3. Remove the file:

3.1 cd /Library/Preferences  (remove the file from where? what will I see on me screen at this point?)
3.2 rm ./com.apple.loginwindow.plist   (is this a second file to remove? Why is there a 3.2 step?)

4. Reboot, typing: reboot

Extra: If mount is Read Only, try "mount -o update /" without quotes (Try it where? Am I typing this onto my screen?)

Why are there steps 2, 2.1, and 2.2,  3.1 and 3.2, etc.? Sorry if I'm sounding completely ignorant, but it doesn't make sense. Could someone please help a less savvy person understand how to follow these steps and what I will see on my screen along the way when I do this? I'm desperate to get my computer up and running again. And I wish Apple would offer some sort of guidance and fix ASAP, not to mention an apology for a horrible upgrade.

pms1962,

This maybe too much for you if you don't know command line, but if you follow the steps it's quite simple.  If you feel uncomfortable or unable to type these commands while in Single-User Mode, then I suggest having a pro take a look at your system.

Try following these steps:

1. Boot to single user mode (hold ⌘ S while powering on the Mac)
2. Type and hit return: /sbin/fsck -fy
3. Type and hit return: /sbin/mount -uw /
4. Type and hit return: rm /Library/Preferences/com.apple.loginwindow.plist
5. Type and hit return: rm /Library/Preferences/loginwindow.plist
6. Type and hit return: rm /private/var/db/.AppleUpgrade
7. Type and hit return: reboot

Thanks very much for your clear directions, abstert!

I was able to follow them with no problem.

However, when I got to #5 and #6, it said "No such file or directory" after I entered each of those commands.

Then when I rebooted, I got the same black screen after I logged in. I should say that my computer boots up fine up to the login screen, where I type in my password under the user name. After that (and a momentary pinwheel), the screen goes black, and all I see is my cursor.

If you could shed any insight or help about that, great! I appreciate you taking the time to explain how to do it. Unfortunately, with those last two commands before the reboot, I think it didn't quite do what it was supposed to do.