A bootable macOS USB stick lets you install or reinstall macOS on a Mac that will not start normally, set up several machines without downloading the installer each time, or do a clean install rather than an upgrade. Apple supports building one with a single command, createinstallmedia, and the procedure has not changed in years. What trips people up is everything around that command: which installer to download, whether the copy you already have is usable, and how to start the Mac from the finished drive — a step that now works completely differently depending on the chip inside.
This page follows the order you actually do it in, quotes Apple’s documentation where the details matter, and flags the three instructions that circulate widely and are wrong.
What you need for a bootable macOS USB
A USB drive with room to spare. Apple’s guidance is specific, and it is not the “16GB minimum” that gets repeated:
A 32GB flash drive has more than enough storage space for any macOS installer, and 16GB is enough for most earlier versions of macOS. If more space is needed, Terminal will let you know.
Note the scoping. 16GB is tied to earlier versions and even then hedged with “most”. Buy on 32GB. Everything on the drive is erased, so start with one you can wipe.
No pre-formatting. A widely repeated instruction says to format the drive as Mac OS Extended (Journaled) or APFS first. APFS is not an option here at all, and the formatting step is unnecessary — Apple states the drive “will automatically be erased using the appropriate format: Mac OS Extended (Journaled).” Erasing it yourself in Disk Utility is a recovery step for when createinstallmedia cannot do it, not a prerequisite.
A recent enough Mac to build on. Apple’s constraint: “if you’re downloading macOS in order to make a bootable installer, make sure that your Mac is using macOS Mojave 10.14 or later.” The Terminal download route below wants Catalina 10.15 or later.
Download the installer
Every bootable macOS USB starts from an installer application, which lands in /Applications like any other app. You can get it from the App Store, or from the Terminal — and the Terminal route is worth using because it tells you what is actually available rather than making you guess a version number:
softwareupdate --list-full-installersThat prints the macOS versions available for your Mac. Take a version string from that output and pass it back:
softwareupdate --fetch-full-installer --full-installer-version VERSION_FROM_THAT_LISTDo it in that order rather than copying a version number out of a guide. Any page that hardcodes a number in the download command is scheduling its own expiry — the number was right when it was written and silently wrong afterwards. Reading it off the live list costs one extra command and never goes stale.
If the installer opens by itself when the download finishes, quit it. You want the application sitting in /Applications, not running.
Check the installer before you type sudo
An application in /Applications with the right name and a plausible size is not necessarily a working installer. A partial download can leave something that looks entirely convincing. On the Mac used for this page, a copy of Install macOS High Sierra.app takes up 4.4 GB and contains exactly one file:
/Applications/Install macOS High Sierra.app
/Applications/Install macOS High Sierra.app/Contents
/Applications/Install macOS High Sierra.app/Contents/SharedSupport
/Applications/Install macOS High Sierra.app/Contents/SharedSupport/InstallESD.dmgNo Contents/Resources, no createinstallmedia, nothing to run. The name is right, the size is right, and the command you are about to type does not exist. One read-only check settles it before you involve sudo:
ls "/Applications/Install macOS Tahoe.app/Contents/Resources/createinstallmedia"Substitute your version’s name. If the path does not resolve, the download is incomplete and building a bootable macOS USB from it will fail. Apple’s remedy for the related “installer does not appear to be a valid installer application” message is to “delete the installer, use Disk Utility to repair your startup disk, then download the installer again.”
Create the bootable macOS USB
Two things before the command that writes the bootable macOS USB. Connect the drive, and rename it to MyVolume — every command Apple publishes hardcodes that name, so renaming the drive is easier than editing the path and getting it wrong. Then confirm nothing else is mounted under a name you might hit by accident.
The command itself, for macOS Tahoe:
sudo /Applications/Install\ macOS\ Tahoe.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolumeApple publishes ten of these commands and nine are that one string with the name swapped: Tahoe, Sequoia, Sonoma, Ventura, Monterey, Big Sur, Catalina, Mojave, High Sierra — all of the form Install macOS NAME.app. El Capitan is the exception twice over. Its application is Install OS X El Capitan.app, because 10.11 predates the rename from OS X to macOS, and it is the one command Apple prints with an extra flag attached.
That extra flag, --applicationpath, causes more confusion than any other part of this. It is commonly explained as being needed “for older versions of macOS”, meaning the version you are writing to the stick. That is not the rule. Apple states it in the troubleshooting notes rather than beside the commands:
If creating the bootable installer on a Mac that is using macOS Sierra 10.12.6 or earlier, append –applicationpath to the command, followed by the appropriate installer path, similar to what is shown at the end of the command for El Capitan.
The trigger is the macOS version of the Mac you are typing on, not the version going onto the drive. A modern Mac building a High Sierra installer does not need it. A Mac running Sierra 10.12.6 building a Sequoia installer would. Apple prints the flag once, on El Capitan, as a shape to copy — a formatting example, not a fact about El Capitan.
Ten commands is also the whole supported list, which is worth knowing when you see a guide promising every macOS version back to Mavericks. Mavericks and 10.9 appear nowhere in Apple’s instructions, and its download page offers disk images for 10.7, 10.8, 10.10, 10.11 and 10.12 — skipping 10.9 in the middle of the run.
When you run the command it asks for your password, then asks you to type Y to confirm erasing the volume. Then it copies, and it takes a while. Leave it alone until the prompt comes back.
Start the Mac from your bootable macOS USB
This is the step where old instructions do the most damage, because the procedure forks at the chip and the two branches share no keystrokes at all. “Restart and hold Option” is correct on an Intel Mac and does nothing on Apple silicon.
Mac with Apple silicon. Shut down. Connect the bootable macOS USB directly to the Mac. Then press and hold the power button — one continuous press from a powered-off machine, keeping your finger down while the Mac starts up underneath it, until startup options appear. Apple’s wording: “As you continue to hold, your Mac starts up and loads startup options, which shows your bootable volumes, including the bootable installer.” Select the installer and click Continue.
Two details people get wrong here. It is not a press to switch on followed by a second press. And Apple gives no duration — any guide handing you a number of seconds invented it.
Any other Mac. Shut down, connect the drive, turn the Mac on and immediately press and hold the Option (Alt) key. Release it when a screen of bootable volumes appears, then select the installer and press Return.
Intel Macs with the Apple T2 Security Chip have one more gate, and it is the usual reason a correctly built drive refuses to appear. Apple’s note for when you “can’t start up from the bootable installer”: “make sure that Startup Security Utility is set to allow booting from external or removable media.” That setting is off on some configurations by default, and no amount of rebuilding the stick will work around it.
One flag to leave alone
--nointeraction turns up in a lot of copy-paste one-liners, advertised as skipping confirmation prompts. It is worth understanding what that means before using it.
The prompt it suppresses is the one asking you to type Y to erase the volume — which is the only thing standing between a mistyped --volume and an erased disk. createinstallmedia runs under sudo and erases whatever that path resolves to. Suppressing the confirmation is defensible inside a deployment script where the target is computed and checked beforehand. Typing a path by hand, it removes the single moment at which a typo is still recoverable.
Worth adding: the flag appears nowhere in Apple’s documentation for this procedure. That does not make it fake, but it does mean its exact behaviour is undocumented, and confirming it would require running a command that erases a disk.
When the bootable macOS USB does not work
Three failures cover most bootable macOS USB problems, and they point in different directions.
- Startup options appear, your installer is not listed. The drive is the problem, not the key you pressed. Rebuild it, and check the
createinstallmediapath first. On a T2 Mac, check Startup Security Utility as well. - The Mac starts up to a circle with a line through it. The bootable macOS USB booted. The macOS on it is not compatible with that Mac, and rebuilding the same version will not change that.
- “Installer does not appear to be a valid installer application.” The download, not the drive. Delete the installer, repair the startup disk with Disk Utility, download it again.
An honest note about scope. Everything above is either Apple’s documented behaviour or something checkable without erasing anything. Whether a particular bootable macOS USB starts a particular Mac is not knowable in advance — that answer arrives when you hold the power button, or Option, and look.
This page previously carried three claims that Apple’s documentation does not support: that --applicationpath is keyed to the macOS version being written, that 16GB is the requirement, and that holding Option works on every Mac. All three are corrected above.
Quick reference
| Step | What to do |
|---|---|
| Drive | 32GB, no pre-formatting, renamed to MyVolume |
| See available versions | softwareupdate --list-full-installers |
| Download one | softwareupdate --fetch-full-installer --full-installer-version VERSION |
| Verify it is complete | ls "/Applications/Install macOS NAME.app/Contents/Resources/createinstallmedia" |
| Build it | sudo /Applications/Install\ macOS\ NAME.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume |
| Boot, Apple silicon | Shut down, then press and hold the power button until startup options appear |
| Boot, Intel | Turn on, immediately hold Option, release at the volume picker |
| Drive not listed on a T2 Mac | Startup Security Utility must allow external media |
The short version: read the version off --list-full-installers rather than a guide, check that createinstallmedia exists before typing sudo, name the drive MyVolume, and remember which key your Mac wants. A bootable macOS USB built this way works on every Mac the installer itself supports.