Upgrading to Golden Gate OS on my iMac 24" M1 Desktop

Your 24-inch M1 iMac (2021) is listed by Apple as compatible with macOS 27 Golden Gate. (Apple Support)

Before upgrading, make sure your important files are backed up and that you have sufficient free storage for the installation. 

Apple recommends backing up before installing a new version of macOS. (Apple Support)

Since you are running macOS Tahoe 26.7, open  > System Settings > General > Software Update and check whether macOS 27 is offered. 

If it is not shown, restart your iMac and check again. 

Apple notes that Software Update only displays software that is compatible with your Mac. (Apple Support)

If Golden Gate still does not appear, Terminal can provide a more specific check. Open Terminal and enter:

softwareupdate --list-full-installers

Look through the results for a macOS 27.x installer. This will tell us whether Apple's software-update service is making a full macOS 27 installer available to your Mac.

If you see a macOS 27.x installer, don't install it yet. 

Paste the complete Terminal output here so we can determine why it is not appearing in Software Update.

If no macOS 27 installer is listed, that gives us a different direction for the next step.

You can find Apple's current guidance here: Apple — How to upgrade to macOS 27 Golden Gate, Apple — How to download and install macOS, and Apple — Identify your iMac model.

Your Terminal output shows that Apple’s software-update service is offering macOS 27 Golden Gate 27.0 to your Mac:

Title: macOS 27 Golden Gate

Version: 27.0

Build: 26A428

Deferred: NO

Apple's current compatibility information lists the iMac (24-inch, M1, 2021) as compatible with macOS 27. (Apple Support)

Apple also documents a Terminal method for downloading a complete macOS installer. Since your --list-full-installers command shows version 27.0 as available, you can try:

softwareupdate --fetch-full-installer --full-installer-version 27.0

Apple states that this command downloads the macOS installer to the Applications folder and displays download progress in Terminal.

When the download completes successfully, you can open the installer from Applications and follow the onscreen instructions. (Apple Support)

I would therefore try the Terminal command above rather than changing system settings or deleting Software Update files.

Apple's normal upgrade procedure is through System Settings > General >> Software Update. (Apple Support)

Apple: How to download and install macOS

Turned off low data mode for wireless settings, used terminal to discover my Mac is compatible with Golden Gate OS 27 and used disk aid in safe mode. I don't know what action triggered the possible upgrade but something changed in Software settings.

Now in Software Settings Golden Gate OS 27 is ready to upload.

I hope it works and the update wheel does not endlessly spin round.

Many thanks guys for your help!

Long shot, but within your Wi-Fi settings, within your current network, is the setting 'Low Data Mode' on? If it is I recommend turning the option off and try again.

Either that or use Terminal (worked for me).

Type in via Terminal:

sudo softwareupdate --list (to see list of available updates)

sudo softwareupdate -i <name of update from the above command>

Restart the iMac and try again.

If that doesn't help..?

Restart in Safe Mode and try again.

Start up your Mac in safe mode - Apple Support

If you are getting an error while attempting to upgrade..?

If an error occurred while updating or installing macOS - Apple Support

Last login: Tue Sep 22 11:42:07 on ttys000

davidlumb@Davids-iMac ~ % softwareupdate --list-full-installers

Finding available software

Software Update found the following full installers:

* Title: macOS 27 Golden Gate, Version: 27.0, Size: 17969056KiB, Build: 26A428, Deferred: NO

* Title: macOS Tahoe, Version: 26.7, Size: 17951133KiB, Build: 25G229, Deferred: NO

* Title: macOS Tahoe, Version: 26.6.2, Size: 17953734KiB, Build: 25G83, Deferred: NO

* Title: macOS Tahoe, Version: 26.6.1, Size: 17938060KiB, Build: 25G76, Deferred: NO

* Title: macOS Tahoe, Version: 26.6, Size: 17942836KiB, Build: 25G72, Deferred: NO

* Title: macOS Tahoe, Version: 26.5.2, Size: 17875271KiB, Build: 25F84, Deferred: NO

* Title: macOS Tahoe, Version: 26.5.1, Size: 17872095KiB, Build: 25F80, Deferred: NO

* Title: macOS Sequoia, Version: 15.8, Size: 15296950KiB, Build: 24H23, Deferred: NO

* Title: macOS Sequoia, Version: 15.7.9, Size: 15289021KiB, Build: 24G830, Deferred: NO

* Title: macOS Sequoia, Version: 15.7.8, Size: 15288760KiB, Build: 24G824, Deferred: NO

* Title: macOS Sequoia, Version: 15.7.7, Size: 15288576KiB, Build: 24G720, Deferred: NO

* Title: macOS Sonoma, Version: 14.8.9, Size: 13334756KiB, Build: 23J631, Deferred: NO

* Title: macOS Sonoma, Version: 14.8.8, Size: 13333989KiB, Build: 23J620, Deferred: NO

* Title: macOS Sonoma, Version: 14.8.7, Size: 13336094KiB, Build: 23J520, Deferred: NO

* Title: macOS Ventura, Version: 13.7.8, Size: 11919053KiB, Build: 22H730, Deferred: NO

* Title: macOS Monterey, Version: 12.7.6, Size: 12118346KiB, Build: 21H1320, Deferred: NO

* Title: macOS Monterey, Version: 12.7.4, Size: 12117810KiB, Build: 21H1123, Deferred: NO

* Title: macOS Big Sur, Version: 11.7.11, Size: 12122736KiB, Build: 20G1443, Deferred: NO

* Title: macOS Big Sur, Version: 11.7.10, Size: 12125478KiB, Build: 20G1427, Deferred: NO

davidlumb@Davids-iMac ~ % 

FYI: Golden Gate departs from previous macOS downloads as it also is now bloated with multi-Gigabyte AI data. My download to an M4 Mac Mini Pro was almost 29 GB before installation.