Repair guides and support for 21.5" Intel iMacs by Apple.

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iMac (mid 2011) Shutting off in the middle of booting up

Hello Everyone.

I reformatted the hard drive of our iMac using the disk utility and re-installed the OS.

It worked for a day and now every time we turn it on we get a chime, the Apple logo shows up about half way through the boot up it shuts down (never booting up). I tried to access the disk utility but it doesn't show up. I think the reason for this is because the system doesn't stay on long enough for it to appear. I have tried turning it on safe mode and same thing happens. It stays on for about a minute and then shuts off.

My question is this: Is it possible that this is a hard drive issue or do you think the CMOS battery needs to be changed? Could it be overheating? Is there a test I can do to be sure?

If its a hard drive issue and I replace it, how would I install the new OS into it?


Update (01/13/2017)

Thank you Dan and Mayer for responding to my inquiry so quickly.

I did try safe mode as well but it also turns off half way through loading.

The iMac was working perfectly fine up until I reformatted and re-installed the OS (Sierra).

I failed to mention that I had a tough time erasing the hard drive and it took a few tries to get the OS installed. I remember getting to the setup screen and it did turn off after a couple of minutes. This is why I thought the issue was with the hard drive because maybe the OS was not installed properly.

I mostly work with MS Windows so I am very green with it comes to the Mac hardware.

I do have a Mac Pro so would it be worth it to connect it to that and use Target Mode? Or is my best chance just take it to a Pro and get them to test it for me?

Answered! View the answer I have this problem too

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Sure its worth trying to see if Target Disk Mode will come up on your system and then see if you can connect it to your Mac Pro.


Hello again. I tried an internet recovery and was able to install the original OS (LION) to a USB, and was able to boot from the USB. Does this mean that the problem is with the hard drive?


OK, that does change things!

So the system wasn't shutting down, it just never finishing booting up. See below update for what you'll need to do and some other guidance.


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2 Answers

Chosen Solution

Power issues tend to be problems with the power supply.

In this case you could have a thermal issue within the heat pipe so the system goes into shutdown so it doesn't damage things from overheating. But this is just a guess without doing the deeper analyst.

Sadly, this may be something a pro will need to handle to figure things out. You'll need to open the system and test the power supply as well as the main logic board. Do you have the needed tools and skills?

Update (01/14/2017)

OK, so the system is working! Good!

So the focus is now shifted to the drive. Here is the IFIXIT guide to replace the drive: iMac Intel 21.5" EMC 2428 Hard Drive Replacement.

As to what to put in, I would put in a hybrid drive (SSHD) which offers some zip over the old HD you had. This is what I go with: Desktop SSHD. A 1 or 2 TB drive should be more than enough for most people.

You do have a second issue which you need to deal with here as the Apple drive is a modified drive which allows the internal thermal sensor to communicate with the systems management system (SMC) which controls the systems cooling. Here we need to use an external thermal sensor to replace the now missing one. This is what I would use: OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade

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Thank you so much. I will buy the parts and I should be able to do it myself :)


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Most Helpful Answer

Half way through is about where the GPU drivers kick in. Try starting in "Safe Mode" by holding down the Shift key on start up and let us know your results.

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