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

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.

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.

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.

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.