My iMac has stalled while installing macOS 15.1…. What do I do?

Press and hold the power button to power off your Mac.

Press the power button to restart your Mac.

If Sequoia hasn’t been installed, try installing it again.

If you can’t install Sequoia from Software Update, search for it on the App Store and install it from there.

If that doesn’t work, boot your Mac in safe mode and try again. When it has been installed and set up, reboot normally.

Just to underscore the advice from PRP_53: Wait it out if you can. On 3 different machines (2 apple silicon and one with intel) all installs/updates involving Sequoia appeared to stop doing anything with "30 minutes remaining", but eventually installed fine.

Be patient, have a cup of you favourite beverage and walk away

If it were to still be in the current state after about 1 hour, then worry

Manually Shutdown the computer via the Power Button

Pause maybe 1 minute

Restart the computer in Safe Mode.

This may or may not allow the installation to complete

Hopefully, successfully

Just to add to what others have mentioned.

Updated 1 M1, 1 M2 and 1 M3 machines each from Sequoia 15.0.1 to current 15.1

On Friday this week the M4 Machine arrives with a Special Version on Sequoia 15.0 uniquest to the M4 machines

I anticipate there will be an updated offered for version 15.1

I also will be anticipating the similar nail bitting periods iso you are not alone in this

The installation process will commence and do a soft reboot several times

If not mistaken, the last soft reboot is the nail bitting one.

It is where the loading bar will progress and pause and seem to hang but given enough time, it should eventual lload the Desktop with the Log In presented

Now a word of some caution and only you know the answer to this.

If there has been some Third Party Security Software installed before the Installation process began..

Cross your fingers and hope for the best

As some people say, patience is a virtue.