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Update Hangs: Late 2015 21.5" iMac from High Sierra to Catalina or Monterey

I'm trying to update my uncles 21.5" late 2015 iMac from High Sierra to Monterey which is supposed to be the last OS it will work with. When that failed, I tried Catalina but met with the same fate. I updated my wife's mid 2015 MBP to Monterey without any issues so thought I'd do the same for my uncle. This is what I did.. ..


Via an Ethernet connection.. ..

  1. Disconnected all external devices
  2. Ran Disk Utility to verify there were no issues with the internal hard drive
  3. Found a link to Monterey on the Apple App Store (you can no longer find it by searching the App Store, it appears they have taken it down but you can find a link directly to it)
  4. Downloaded the install Monterey app, it was something like 45gig if I recall
  5. Ran the install Monterey app
  6. Got the typical install screen and progress bar and let it run...takes around a half hour or more as the progress bar moves to the right to the end of its travel and then sits there with less than a minute to complete
  7. It stayed at less than a minute to complete for a good 30-40 minutes until I finally shut down via the power button on the back of the iMac
  8. Pressed the power button to restart it and got the black screen with the install progress bar and said it had about 26 minutes to go so I let it go but the same thing happened when it got to the end of the progress bar.
  9. I did this with Catalina too, got the same results. There is no indication at this point if anything is happening or not
  10. I finally gave up and reinstalled High Sierra via his backup drive. I need to load a more recent OS since Safari no longer has compatibility with many of the financial websites he uses and High Sierra no longer gets security updates. Since Apple states his iMac is compatible with Monterey, why won't it complete the update?


I've read articles about this and did in fact try going the Safe Boot route but to no avail. I did a PRAM reset to no avail. I was wondering if it might be a certificate date issue where Catalina or Monterey OS on the Apple servers is looking for a newer iMac but Apple says it should work.


Help please.

Posted on Jul 18, 2023 12:24 PM

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Posted on Jul 23, 2023 9:54 AM

Finally.... Success! After waiting a couple of hours with 'less than a minute remaining' using a 2015 MBP, an error message was displayed. Can't quite recall what it said but that at least was better than simply waiting and waiting with nothing ever changing. So, I reset PRAM on that MBP and used the bootable installer again and this time it worked.

I don't know why PRAM needed reset, I've read where others have had success in doing so but have not read why it needs to be done. If anyone knows what changes are caused by a reset that then allows a successful install, pls reply. So now MacOS Monterey is loaded on the SSD and the next step is for me to remove the spinning hard drive in my uncles iMac and replace it with this SSD. For those who also want to do this, you should buy the proper kit that comes with a mini pizza roller cutter used to cut through the adhesive that holds the screen on and it comes with new tape.

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Jul 23, 2023 9:54 AM in response to Bgnmarty

Finally.... Success! After waiting a couple of hours with 'less than a minute remaining' using a 2015 MBP, an error message was displayed. Can't quite recall what it said but that at least was better than simply waiting and waiting with nothing ever changing. So, I reset PRAM on that MBP and used the bootable installer again and this time it worked.

I don't know why PRAM needed reset, I've read where others have had success in doing so but have not read why it needs to be done. If anyone knows what changes are caused by a reset that then allows a successful install, pls reply. So now MacOS Monterey is loaded on the SSD and the next step is for me to remove the spinning hard drive in my uncles iMac and replace it with this SSD. For those who also want to do this, you should buy the proper kit that comes with a mini pizza roller cutter used to cut through the adhesive that holds the screen on and it comes with new tape.

Jul 18, 2023 2:09 PM in response to Bgnmarty

Hi there!



Try to run First Aid, then upgrade to macOS Monterey from Internet Recovery. Make sure you still have an up to date backup prior to this procedure.



To begin, startup into Internet Recovery mode: Use macOS Recovery on an Intel-based Mac - Apple Support.


1. Restart your Mac.
2. Immediately press and hold Option-Command-R to start up from macOS Recovery over the internet. Use this key combination to reinstall macOS and upgrade to the latest version of macOS that’s compatible with your Mac.



Next, from the utilities window in macOS Recovery, select Disk Utility and click Continue. You can now run First Aid on the internal disks. This article can help out with that: How to repair a Mac disk with Disk Utility - Apple Support.



After First Aid, quit Disk Utility to return to the utilities window. Then, select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions to upgrade to Monterey.



Let us know if this works for you!


- Jack

Jul 18, 2023 3:18 PM in response to Jack-19

Hi Jack,

Thanks for your reply, I should have included that in my initial write up, been there, done that, didn't work. Got the same issue at the same point in time...less than a minute remaining. BTW, any time I ran First Aid, whether via the Internet Recovery or local to the iMac, it never found any issues, always passed with the green checkmark, never listed that it was repairing something.

Jul 18, 2023 3:37 PM in response to Jack-19

I have not tried that yet. I'm thinking about ordering a new SSD to replace the spinning drive in my uncle's late 2015 iMac and if I do that, I can try loading Monterey at my house and then take it to him (1.5hrs away) and replace his drive. Not only would he get Monterey, he'd get a nice speed boost to keep his iMac alive for a few more years.

If loading Monterey on that SSD by normal means (the installer app) doesn't work, I would create a bootable installer but I seem to recall reading somewhere that to do this, I would need to create it on a Mac that Monterey would work on, is that correct? My Mac is M1 Pro based but I could use my wife's 2015 MBP to do it.

Jul 22, 2023 1:55 PM in response to Jack-19

Ok, I created a bootable installer following the directions from your link and was successful doing that. Then I ordered a new SSD that I will be putting in my uncles iMac in place of the OEM spinning hard drive. I plugged in the Crucial SSD, formatted it using Disk Utility to Mac OS Extended Journaled using the GUID Partition Map and erased and formatted it. Then ran First Aid on the disk and its partition, both passed.

I used my wife's MBP running Monterey and restarted it, pressing the Option key and selected the Monterey installer and directed it to be installed on the new SSD. The results so far are the same as before, it gets to less than a minute remaining and seems to hang there. It's been that way for over an hour now. Really not sure what to do next - very frustrating.

Update Hangs: Late 2015 21.5" iMac from High Sierra to Catalina or Monterey

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