How to Create a Bootable macOS Installer on a USB Drive

A bootable USB installer is essential for a clean macOS install, downgrading, or troubleshooting a Mac that won’t boot into its normal system at all. This uses Apple’s own built-in createinstallmedia tool — no third-party software required.

Step 1: get a USB drive of sufficient size

You’ll need at least a 16GB USB drive, formatted and ready to be fully erased — createinstallmedia will overwrite everything on it.

Step 2: download the full macOS installer app

Open the App Store, search for the macOS version you want (e.g., “macOS Sonoma”), and download it. This places a full installer application in /Applications — if it opens automatically once downloaded, quit it without proceeding through the actual install, since you only need the installer app itself, not to run it yet.

ls /Applications/ | grep "Install macOS"

Step 3: identify your USB drive’s disk identifier

diskutil list

Find your USB drive in the output (check the size to confirm) and note its identifier, typically something like /dev/disk4.

Step 4: erase and format the USB drive appropriately

diskutil eraseDisk JHFS+ MyInstaller /dev/disk4

This names the volume “MyInstaller” and formats it as Mac OS Extended (Journaled) — the format createinstallmedia expects.

Step 5: run createinstallmedia

sudo /Applications/Install\ macOS\ Sonoma.app/Contents/Resources/createinstallmedia --volume /Volumes/MyInstaller

Adjust the app name in the path to match whichever macOS version you downloaded in step 2. You’ll be prompted to confirm erasing the volume (again) and will need to enter your password for the sudo command.

Step 6: wait for it to complete

This process copies several gigabytes and takes meaningful time — expect anywhere from 10 to 30+ minutes depending on your USB drive’s write speed. The terminal will show progress percentages through several stages (“Erasing disk,” “Copying installer files,” “Making disk bootable”).

Step 7: boot from the finished installer

On an Apple Silicon Mac: shut down completely, then press and hold the power button until “Loading startup options” appears, and select the USB installer volume.

On an Intel Mac: restart while holding the Option (⌥) key, and select the USB installer volume from the boot picker that appears.

Step 8: use the installer for your intended purpose

Once booted from the USB installer, you have access to Disk Utility (to erase or repair the internal drive first, if doing a clean install), and the installer itself, which can either install fresh or, in some recovery scenarios, repair an existing installation without erasing user data.

Why build this locally instead of just using Recovery Mode

Built-in Recovery Mode (holding Cmd+R at startup, or the Apple Silicon equivalent) downloads its installer over the internet at boot time and is genuinely useful for most routine reinstalls — a dedicated USB installer is worth having specifically when you need a specific macOS version (rather than whatever Recovery defaults to), need to install on a Mac with no working internet connection, or want to keep a permanent, offline installer ready for multiple machines without re-downloading it each time.

Step 9: Apple Silicon’s Startup Security Utility affects whether external boot is even allowed

Apple Silicon Macs ship with a Startup Security Utility policy (accessible from Recovery Mode) governing whether the machine will boot from external, non-Apple-signed media at all — by default this is permitted, but a Mac with its security policy tightened (via Reduced Security disabled, or Full Security enforced) may refuse to boot your USB installer entirely regardless of how correctly it was built. If a properly-created installer USB simply doesn’t appear as a boot option on an Apple Silicon Mac, checking this policy in Recovery Mode’s Startup Security Utility — rather than assuming the USB itself is faulty — is the correct next diagnostic step, since this security boundary didn’t exist at all on Intel Macs in the same form.

Step 10: verify the installer app itself before building media from it

codesign -dv --verbose=4 "/Applications/Install macOS Sonoma.app"
spctl -a -vv "/Applications/Install macOS Sonoma.app"

Since createinstallmedia faithfully copies whatever’s inside the installer app you point it at, confirming that app’s own code signature and notarization are intact before spending 10-30 minutes building the USB is a cheap sanity check — particularly relevant if the installer was downloaded from anywhere other than the App Store directly, or if the download process was interrupted and silently resumed in a way that could have left the app bundle subtly incomplete.

Step 11: reuse the same USB drive for a different macOS version later

Running createinstallmedia again against the same USB drive, pointed at a different installer app, fully erases and rebuilds it from scratch — there’s no incremental update path, and no need to manually erase the drive first, since the tool’s own erase step (confirmed again during the run) handles that as part of the process. Keeping a single dedicated USB drive relabeled and rebuilt as needed, rather than accumulating a separate physical drive per macOS version, is the more practical long-term approach for anyone who builds these more than very occasionally.

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