How to Create a Bootable macOS Installer on a USB Drive
A complete walkthrough using the built-in createinstallmedia tool to build a bootable USB installer for a clean install or major troubleshooting.
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 listFind 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/disk4This 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/MyInstallerAdjust 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.