My iMac has a broken screen which doesn't allow me to view anything. So I got a monitor and connected it to the Mac and it works great. My problem is when I boot up my iMac it won't display the grey screen with the apple logo loading into the OS. It simply stays black until it's at the login sequence of the boot. How can I make my iMac be able to display the whole booting process? I tried using the Mirror Displays and that did not work for me. I also read if I don't select the mirror option it should work, but that did not work for me either. I want to able to factory reset my iMac but I can't do it because I can't see the booting part of my computer which is where I get to partition the drive and wipe it clean. Any thoughts or suggestions on how I can fix this?

  • My guess would be that if you took the iMac apart and disconnected the internal monitor, then computer would automatically switch to the external monitor as the main display. – David Anderson Sep 17 '16 at 8:50
  • I thought it had to do with the fact that I had my monitor in auto detect so it would alternate between hdmi and the other options in the monitor but that was not the case I specifically set it to boot into hdmi only and I still can't see the boot process.I will try to disconnect the internal monitor like you said. Is that the only way to wipe your drive clean? In the boot sequence is there any other way to wipe a hard drive other than when it boots up? @DavidAnderson – Katz Sep 18 '16 at 3:45
  • What is the model year and screen size of your iMac? What type of external monitor are you using? (i.e. VGA, HDMI or DVI) – David Anderson Sep 18 '16 at 8:08
  • I'm using a 20 Inch early 2009 iMac running El capitan, and my external monitor is an NVIDIA Geoforce 24 inch. I connected the monitor with a thunderbolt to HDMI cable. – Katz Sep 22 '16 at 1:09
  • The 20 Inch early 2009 iMac does not have a Thunderbolt port. It does have a Mini DisplayPort. NVIDIA Geforce is not a monitor, but rather is a graphics processing unit (GPU) inside the iMac. – David Anderson Sep 22 '16 at 11:53

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