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I have a 2018 MacBook Pro (MacBookPro15,2) that originally came with Mojave which currently has MacOS Sonoma installed. I have another installation of High Sierra (10.13.6) on an external SSD that I would like to run on the laptop by booting from the external drive. I have allowed booting from an external medium and disabled boot security as described in Startup Security Utility on a Mac with an Apple T2 Security Chip.

However, whenever it tries to boot from the actual external drive I get the prohibitory boot sign even though in theory 10.13.6 should be compatible with a 2018 MacBook Pro.

I have verified that the external drive is bootable as I can boot it from a virtual machine, it seems like the boot-loader on the system does not accept as a valid version.

Is there something else that I could try to make the drive boot?

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  • You can boot to Internet Recovery by holding down the Shift-Option-Command-R keys. Once booted, you can open a Terminal application window and enter the command sw_vers to get version and build numbers. This would be for the earliest version of macOS that Apple currently offers for your Mac. I wonder if you will get High Sierra or Mojave? This might give insight to why you can not boot the external drive. Nov 24, 2023 at 20:01

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The Mac needs a specific build of High Sierra.

10.13.6 was released on July 9, 2018 as build 17G65. The 2018 MBP was also released in July of that year, but needs build 17G2112, with additional drivers, specifically for that hardware.

Mojave was released in September 2018.

If you're sure you have the correct build, then perhaps the factory model with Mojave has updated firmware that precludes High Sierra.

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  • Do you know where to download build 17G2112? The ones in the App Store do not seem to be compatible with a 2018 Macbook Pro. Nov 25, 2023 at 2:15
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    @user1628056 It may not be readily available. What do you need High Sierra for, that can't be done with Mojave?
    – benwiggy
    Nov 25, 2023 at 9:36

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