Create bootable legacy macOS USB installers on Apple Silicon

What this is

If you’re using a new Apple Silicon Mac (M1 / M2 / M3) and need to install macOS on an older Intel Mac, Apple makes this surprisingly difficult — sometimes impossible — using official tools.

Older macOS versions are distributed as .pkg installers, but:

• They don’t produce a bootable USB on modern Macs
• createinstallmedia often fails or isn’t accessible
• The installer app can’t be launched on Apple Silicon

This script exists to solve that exact problem.

It takes an official macOS installer .pkg, extracts it correctly, and creates a bootable USB installer that works on Intel Macs, even when created on Apple Silicon.

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When you need this

Use this script if:

• You only have access to a modern Apple Silicon Mac
• You need to reinstall macOS on an older Intel Mac
• Internet Recovery doesn’t work or isn’t available
• The macOS installer you downloaded is a .pkg, not an app

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What it does

The script automates a process that normally requires undocumented manual steps:

1. Extracts the hidden Install macOS.app from Apple’s .pkg
2. Mounts the required disk images
3. Runs Apple’s own createinstallmedia in a way that still works on modern macOS
4. Produces a properly bootable USB installer for Intel Macs

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Requirements

• macOS (Apple Silicon or Intel)
• USB drive (8 GB minimum, 16 GB recommended)
• Official macOS installer .pkg
• Administrator (sudo) access

Legacy macOS installers can be downloaded from Apple:
https://support.apple.com/102662

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Usage

1. Clone the repository

git clone https://github.com/Raasu2/macOS-BootableUSB-Builder.git
cd macOS-BootableUSB-Builder

2. Make the script executable

chmod +x create_bootable_installer.sh

3. Insert your USB drive

Note the volume name (example: InstallUSB).

⚠️ All data on this drive will be erased.

4. Run the script

./create_bootable_installer.sh

5. Follow the prompts

You’ll be asked for:

• The full path to the macOS installer .pkg
• The USB volume name

The script handles the rest.

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Booting the Intel Mac

1. Insert the USB into the Intel Mac
2. Power on and hold Option (⌥)
3. Select the installer and proceed normally

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Notes & safety

• The script only erases the USB volume you specify
• Temporary files are cleaned up automatically
• Double-check the USB name before confirming

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Background

This script is based on real recovery steps discussed here:
https://apple.stackexchange.com/questions/418100/create-an-el-capitan-rescue-usb-using-a-modern-m1-mac

The manual method works — this script makes it repeatable and less error-prone.

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License

MIT License