Monterey won't install on late 2015 iMac

I have basically the same machine as you except with a 3.1 GHz processor. I've the last possible upgrade of Monterey installed but it was difficult and time consuming. It runs good now on my machine but does have a couple glitches with time machine backup (working but not manually) and iPhone backup (?).

I've learned to be very patient, don't mess with process and don't believe what it says as to time remaining, the longest minute of my life I think. Ten minutes can be a hour and a half or more. Every update since originally installed has been this way, sometimes ending in a crash failure with a difficult restart. Usually because I or something else interfered in the process I think. (Non-Mac security software or login items can do that.). Since the MacOS file system change it seems the upgrades then updates have been progressively worse like this.

With patients my last update went very smoothly though. I think it was because I connected with an Ethernet cable (not Wifi) then turned off power saving and screen saver functions so the install process couldn't be interrupted in any way. I've also learned to hardwire my keyboard and mouse to the machine and not rely on bluetooth. Sometimes the status bar doesn't seem to move at all so I put my mouse pointer at it's end to see if anything is happening. The bar will freeze for 20 minutes or so until it moves a tiny bit, then it will take off and stop again then sit. It can go through a few different status bar cycles like that then eventually the login window pops up just as all hope is being lost. I'm telling you this to underscore "PATIENCE".

What dialabrain tells you is good advice. I've done all that in the past too but the biggest thing for me I think was just to be patient with the process. It can be hard to believe it's taking this long and something hasn't happened to kill the install.

Good Luck

There are a number of reasons that could happen.

Not enough space on the hard drive - Make sure you have at least 40-50GBs free.

A problem with the hard drive - Boot into Recovery Mode and run First Aid from Disk Utility

Something installed interfering with the update - try while ib Safe Mode.

Has the Original Internal Drive been replaced with an After Market Drive ?

Predicated on Yes.

An Apple Original Drive would need to be the Destination for Monterey successfully install.

The EFI Boot ROM contained in the Monterey Installer will verify and if it finds the Drive is not Apple Original, it may fail to install

The EFI Boot ROM must Flash the Computer ( not the Drive ) with a newer EFI Boot ROM

The workaround that has worked is to Swap Out the After Market Drive and replace with the Apple Original

Run the Installer to that drive, wherein the Flashing of computer should occur

The reverse the previous Swap step and run the Installer to that drive

EDITED

bookmanjb wrote:

If it were not compatible, wouldn't there be some sort of alert at the beginning of the process instead of simply hanging at the very end?

You are correct. If it was an aftermarket drive issue it would have errored out long before.

bookmanjb wrote:

thanks for your answer.

Welcome

The original internal hard drive is still installed.

From the System Report:

 Capacity: 1 TB (1.000.204.886.016 bytes)

  Model: APPLE HDD HTS541010A9E662               

  Revision: JA0AB5D0

If it were not compatible, wouldn't there be some sort of alert at the beginning of the process instead of simply hanging at the very end?

With the above additional and previously missing information

" Has the Original Internal Drive been replaced with an After Market Drive ?

Predicated on Yes. "

This becomes no longer relevant

You're welcome. Are you trying to install Monterey on the external SSD?

I have a Late 2015 21.5" iMac and Monterey runs beautifully on it. So it's likely not a compatibility issue.

Just and FYI, the internal drive is now a Samsung EVO 860 SSD.