Mojave installed on an external SSD does not boot on 27 inch iMac with BigSur

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Level 1

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Mojave installed on an external SSD does not boot on 27 inch iMac with BigSur

I installed Mojave on an external SSD to continue using 32 bit software on my new 27 inch iMac 2020 with BigSur.

Unfortunately, I can't boot from this SSD even though I've changed the Boot Security Utility settings to allow booting from external media and disable security so I can boot into a different operating system.

See https://support.apple.com/nl-nl/HT201372

On my 15 inch macbook pro (from 2016) I can use the SSD to boot, but on my iMac I get a symbol of a circle with a diagonal line in it (access denied).

I was advised by Apple to format Mojave as journaled but as soon as the installer installs Mojave the disk is formatted APFS, I have no choice and I don't think this is the problem.

Does anyone know what I can do to use a bootable SSD with Mojave on my 27 inch iMac 2020? Thank you very much in advance!

iMac Pro

Posted on Jul 28, 2021 1:30 PM

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Answer 1

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Eau Rouge

Level 8

42,554 points

Jul 28, 2021 2:40 PM in response to lapuella

An iMac from 2020 will have come with macOS Catalina or macOS Big Sur preinstalled.

Macs cannot boot to older systems than the ones they came preinstalled with.

Therefore you will not be able to boot to this SSD with Mojave installed on it.

If you install virtual machine software on your mac you may be able to boot to the

Mojave drive..

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Answer 2

lapuella Author

Level 1

8 points

Jul 28, 2021 2:00 PM in response to BDAqua

Thank you BDAqua. It is a Sandisk Extreme 1 TB SSD. Yes, disk utility sees the disk and it looks alright. The SSD also shows as a possible boot disk in system preferences. But when I select it and restart, it does not work.

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Answer 3

lapuella Author

Level 1

8 points

Jul 29, 2021 4:36 AM in response to Eau Rouge

Thank you Eau Rouge, that explains why the SSD I made with BigSur works (boots) and Mojave not. Maybe I will install Windows 10 with a virtual machine to use the 32 bit software. It is a pity that there is no other solution, but I am glad I know now why it is not working.

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