Steps to Fix Crashing Security Preferences and Install High Sierra

1. Boot into Safe Mode

Safe Mode can help resolve system issues and may allow you to access Security Preferences without crashing.

• How to Boot into Safe Mode:
  1. Shut down your iMac.
  2. Restart it and immediately press and hold the Shift key.
  3. Keep holding Shift until you see the Apple logo and progress bar.
  4. Once in Safe Mode, try opening System Preferences > Security & Privacy again.

If Safe Mode works, you can allow the app from an "unknown creator" and proceed with your High Sierra installation.

2. Reset NVRAM/PRAM

Sometimes issues like this can be due to corrupt NVRAM settings. Resetting it might fix crashes and give you access to System Preferences.

• How to Reset NVRAM/PRAM:
  1. Shut down your iMac.
  2. Turn it on and immediately press and hold Option + Command + P + R for about 20 seconds.
  3. Release the keys when you hear the startup chime or see the Apple logo appear twice.

After resetting the NVRAM, try accessing Security & Privacy again.

3. Use Terminal to Bypass Security Settings

If you're unable to open Security Preferences via the graphical interface, you can use Terminal to bypass security settings for apps downloaded from an unknown developer.

• Steps to Disable Gatekeeper:
  1. Open Terminal (Applications > Utilities > Terminal).
  2. Type the following command to disable Gatekeeper, which manages app security:

     bash
     Copy code
     sudo spctl --master-disable
     

  3. Press Enter and enter your admin password when prompted.

This command disables the macOS security that blocks apps from unidentified developers. After running this command, you should be able to open and install the High Sierra installer without the unknown creator warning.

4. Update via Terminal

If you can’t resolve the issue via Security Preferences or System Preferences, you might be able to upgrade directly to High Sierra through the Terminal using the softwareupdate command.

• Steps to Update via Terminal:
  1. Open Terminal.
  2. Run the following command to list all available macOS updates:

     bash
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     softwareupdate --list
     

  3. Once you find the update for High Sierra, install it using:

     bash
     Copy code
     softwareupdate --install "macOS High Sierra"
     

  4. Follow the prompts to complete the installation.

5. Reinstall macOS Sierra and Retry

If the above steps fail, you may need to reinstall macOS Sierra to fix any underlying system issues before upgrading to High Sierra.

• Reinstall Sierra via Recovery Mode:
  1. Restart your iMac and hold Command + R to boot into Recovery Mode.
  2. Select Reinstall macOS and choose macOS Sierra.
  3. Once the reinstall is complete, try upgrading to High Sierra again using either the App Store or the Terminal commands mentioned earlier.

Conclusion

If your Security Preferences keep crashing, starting with Safe Mode or resetting NVRAM might resolve the issue. If that doesn't work, disabling Gatekeeper using Terminal should allow you to install High Sierra. You can also update directly via Terminal or, if necessary, reinstall macOS Sierra to fix any potential system corruption.

Let me know if you encounter further issues or need more help!