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I tried clean installing High Sierra in by making bootable USB. I did the following:

  1. PRAM Reset
  2. Pressed Option Key at the Startup
  3. Choose USB with Install High Sierra from the Drives
  4. The Apple logo screen appears and stays endlessly

I thought the USB might be corrupt so used different USB with light indicator and made bootable version of the High Sierra but same thing happens and after selecting install High Sierra the screen with Apple logo appears endlessly. The USB only lits up during the drive selection time and momentarily after Apple logo appears and then the light of the USB stays off indicating the iMac is not reading the USB.

Is this related to bad motherboard?


Log info as suggested by @At0mic & @Allan

enter image description here

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  • 1
    Have you tried reinstalling macOS High Sierra from Internet Recovery or macOS Recovery? This would be the easiest way to proceed.
    – Todd
    May 13 '20 at 13:47
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    Have you tried Internet Recovery? This downloads the macOS Recovery image from the Internet so it might let you into macOS Recovery that way.
    – Todd
    May 13 '20 at 13:56
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    Try holding Shift-Option-Command-R or Option-Command-R. Refer to this for further information. support.apple.com/en-us/HT201314
    – Todd
    May 13 '20 at 13:58
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    You can try pressing and holding 'D' to start the Apple Diagnostics test. This does a basic check and might tell you if the USB ports aren't working.
    – Todd
    May 13 '20 at 14:13
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    If internet recovery doesn't work, also try booting from the USB and immediately holding down cmd-v, as this will show you the log as you boot.
    – At0mic
    May 13 '20 at 14:35
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Is this related to bad motherboard?

Basically, yes.

You’ve got some good diagnostic information from attempting to boot from a USB installer. If both your internal drive and the new USB installer have the exact same symptoms, the problem is with your computer itself.

Now, this particular vintage iMac had a removable GPU “card” and chances are (based on my professional experience), this is likely the case; these would often fail. To get further confirmation, having a look at the boot messages (Verbose Mode - hold Command V while booting) will help determine this. Also, booting to the diagnostic disk that came with your Mac would allow you to run the graphics tests.

Can you replace it?

Absolutely. Unlike 2012 and newer iMac computers that have the display assembly glued in place, the 2009-2011 models used magnets and Torx screws to hold it in place. User servicing is definitely doable here. IFixit.com has excellent repair instructions.

However, these are quite “expensive” and very rare as the 27” iMac was much more popular. A cursory search on eBay found only one at over $100USD at the time of this writing. Is it worth fixing? IMO, no. It’s obsolete and no matter how much effort you put into it, it will only become more difficult to maintain. However, if you’re into “retro computing,” this may be something you don’t want to give up on just yet.

Now, it could still be your logic board, but I think it’s unlikely (the probability isn’t zero). The nice thing with eBay now, is that you get 30 days to return even on used products. So, your risk is very low if it turns out not to be the GPU.

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  • I have updated the boot log in my question.
    – gfdsal
    May 13 '20 at 15:13
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    Yes. Both the USB installer and a full installation use a graphical interface. The fact its failing on the USB tells you its got nothing to do with macOS installed on your machine.
    – Allan
    May 13 '20 at 15:40
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    No, nothing bad inferred or implied. Many people think this is Apple tech support. I wouldn't go though the trouble unless you’ve got a replacement GPU or you intend to part it out and sell on eBay as funds fora new machine
    – Allan
    May 13 '20 at 16:00
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    It should be. The older diagnostics were much more useful than the newer utilities Apple provides.
    – Allan
    May 15 '20 at 14:00
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    I'm hoping it is memory as well, because it's a very easy and inexpensive fix.
    – Allan
    May 23 '20 at 2:08

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