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The login screen to my iMac is now half black every time it starts as shown here. Once logged in, the display is fine and I can view the whole screen. :s

Any ideas why this might be and how to resolve this?

  • Try enabling and disabling Grayscale mode in System Preferences>Accessibility>Displays – JBis Apr 22 '18 at 15:45
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If it is like that under boot, login screen (as shown), and logged in then it is likely that there is a problem with the actual display or the cables that connect the display to the rest of the system.

It could be the video card as well. If you want to be really sure it is not software related try booting from an external drive, if you can.

  • It's only on the login screen. Once logged in, the monitor is okay and displays the full desktop. – Bhav Apr 22 '18 at 12:09
  • I would be tempted to backup your data, wipe the drive and reinstall from scratch if that is the case. Sounds like OS corruption – Steve Chambers Apr 22 '18 at 16:08
  • Hmm. I think this has started since I've reinstalled the OS. Apple support suggested reinstalling the OS as a potential solution to resolving a sleep wake failure I've got on my iMac.reinstall the OS – Bhav Apr 22 '18 at 18:29
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    NOT reinstall. Backup, and wipe the drive. In other words reformat it so there is zero data on it before you reinstall. If the problem returns then, it's hardware. – Steve Chambers Apr 22 '18 at 20:32
  • Formatting the hard drive seems to work. – Bhav Apr 27 '18 at 5:51
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  1. reset Nvram or Pram (Command + Option + P + R)
  2. Choose Apple menu > Shut Down. After your Mac shuts down, unplug the power cord. Wait 15 seconds. Plug the power cord back in. Wait 5 seconds, then press the power button again to turn on your Mac. For Intel-based Xserve computers that aren't responding, you can shut down locally or by using remote commands. You can also press and hold the power button for 5 seconds.

https://support.apple.com/en-in/HT201295

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