External Startup Drive El Capitan (10.11.6) on iMac 27in 2020 - Update Software Issue

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External Startup Drive El Capitan (10.11.6) on iMac 27in 2020 - Update Software Issue

Background:

I have an iMac 2020 (27in) running Big Sir (11.1) on it's main internal drive. I would like to change the startup drive to an old MacOS X (El Capitan, 10.11.6) via an external USB flash drive. As I have some old software I want to run.

The Issue:

The external USB flash stick works fine (starts up) on an older iMac running El Capitan, but is not recognised as a startup disk on the new iMac 2020 running Big Sur, when you go to system preferences> Start up Disks.

Interestingly, the startup disk is recognised and attempts to start up if you hold down the alt key for the boot menu on startup. However I then get stuck in the dreaded cycle of the iMac asking: 'A software update is required to use this startup disk'.

What I've tried:

Changing the T2 boot security settings.
Connecting to a Wifi network on boot screen.
Using different external media (a brand new WD hard disk 4tb, a brand new SanDisk flash drive 32gb).

Is there something else I'm missing?

Thank you.

Posted on Jul 29, 2021 12:54 AM

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Answer 1

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Eau Rouge

Level 8

42,590 points

Jul 29, 2021 7:54 AM in response to vince480

A Mac that comes preinstalled with a newer OS like Big Sur cannot boot to an older OS.

As your Mac is from 2020 it probably came preinstalled either with macOS Catalina or macOS Big Sur.

Therefore you will not be able to boot to th El Capitan install.

You can try installing virtual machine software to run El Capitan but but performance may not be that great.

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External Startup Drive El Capitan (10.11.6) on iMac 27in 2020 - Update Software Issue

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