Hi everyone,
I used the following solution to successfully get my iMac working yesterday. Hope this helps you too
What is important here, is that when you enter these commands
hit return (enter) quickly after typing each command in - otherwise they tend to automatically start running, sometimes halfway through writing them, and the whole process doesn't work. Took me about three attempts to figure out that this was what I was doing wrong initially. So here's what you need to do:
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First, you need to shut down your Mac, if it's powered on. Pressing the power key for a couple of seconds will do the trick.
Next, you have to boot your Mac in single-user mode. That's done by pressing the Command and S keys at the same time, right after pressing the power key. If you did it right, you should see a whole bunch of lines on the screen in what is a Terminal-like interface.
(I've seen someone recommend that you reset the PRAM three times before booting in the single-user mode. There is no harm in doing that. Hit the Command, Option, P and R keys at the same time, right after you power it on, and wait until you hear the startup chime for the third time before letting go. Then, quickly press the Command and S keys, to get you into the single-user mode.)
You then need to type the following commands:
- /sbin/fsck -fy
- /sbin/mount -uw /
- rm -f /Library/Preferences/com.apple.loginwindow.plist
- rm -f /var/db/.AppleUpgrade
- reboot
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http://betanews.com/2016/12/15/blank-screen-cursor-macos-update-fix/