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Original post by: Dan ,

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How about leveraging Internet recovery! Then you can install the older OS onto the drive first so you can then update the firmware Or download the OS installer and then install the older OS onto a USB thumb drive to then run the firmware updater and Disk Utility to prep the drive.

[https://support.apple.com/en-us/HT204904|How to reinstall macOS] using either '''Command (⌘)-R''' or '''Shift-Option-Command-R'''. To prep a USB Thumb drive follow this guide: [https://www.macworld.com/article/3092900/macs/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive]

Keep in mind High Sierra will upgrade the GUID Journaled file system to the newer GUID APFS. If you don’t want that you’ll need to install Sierra and stay with it. Or edit the OS installer script [http://osxdaily.com/2017/10/17/how-skip-apfs-macos-high-sierra/|How to Skip Converting to APFS When Installing macOS High Sierra]

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