To correctly format the USB to create a bootable installation disk that is recognizable to macOS, use specific formats and partition settings. First make sure that the USB is at least 12GB (16GB or greater recommended) and back up the data; then format the USB to "Mac OS Extended (Journaled)" through Disk Tool and set the partition scheme to "GUID Partition Map"; then download the required version of the macOS installer from the Mac App Store; then run the command sudo /Applications/Install\\ macOS\\ [version].app/Contents/Resources/createinstallmedia --volume /Volumes/USB name to create a bootable USB; finally hold down the Option key when restarting the Mac to select USB boot. The order of the above steps cannot be reversed, and it must be operated accurately to avoid accidentally deleting hard disk data.

To properly format a USB for a macOS bootable installer, you need to prepare the drive using specific formatting and partitioning settings that macOS expects. This ensures the installer can recognize and use the USB correctly during installation.

Use a USB with at least 12GB of storage

Before anything else, make sure your USB flash drive has enough space — at least 12GB is recommended, though 16GB or more is better if you're installing a newer version of macOS. The drive will be completely erased, so back up any important data before proceeding.

• Plug in the USB drive
• Open Disk Utility (you can find it via Spotlight or Applications > Utilities)
• Select the USB drive from the list on the left (be sure to pick the correct one!)
• Click “Erase” at the top
• Choose these settings:
  • Name : Something like "MacUSB" or "InstallDrive"
  • Format : Mac OS Extended (Journaled)
  • Scheme : GUID Partition Map

Click “Erase” to confirm. Wait until the process completes.

Download the macOS installer

You'll need a copy of the macOS installer from the Mac App Store. Depending on your needs, this could be the latest version or an older one compatible with your hardware.

• Go to the Mac App Store or search directly in the App Store app
• Find the macOS version you want and click "Get" then "Download"
• The installer will appear in your Applications folder

Note: If you're downloading a version not for your current Mac, you may need to adjust System Preferences to allow apps from unidentified developers.

Create the bootable installer using Terminal

Once the USB is formatted and the macOS installer is downloaded, it's time to create the bootable USB using the Terminal command line tool.

• Open Terminal (again, under Applications > Utilities)
• Type or paste the following command, replacing “YourUSBName” with the actual name of your USB:

 sudo /Applications/Install\ macOS\ [Version].app/Contents/Resources/createinstallmedia --volume /Volumes/YourUSBName

For example, if your USB is named “MacUSB” and you're installing Monterey:

 sudo /Applications/Install\ macOS\ Monterey.app/Contents/Resources/createinstallmedia --volume /Volumes/MacUSB

• Press Enter and type your password when prompted (you won't see it as you type)
• Confirm by typing “Y” when asked
• Wait several minutes while the tool copies the installer files to the USB

This process will erase the USB again and make it bootable.

Boot from the USB

After the process finishes, your USB is ready to use. To test or install macOS:

• Insert the USB into your Mac
• Restart the Mac
• Immediately hold down the Option (?) key after restart
• You should see the USB drive labeled “macOS Install Media” — select it with the arrow keys and press Enter

If everything was done right, the macOS installer will load from the USB.

That's the basic process. It's straightforward once you've got the steps down, but easy to mess up if you choose the wrong disk or skip a formatting step. Double-check everything before hitting “erase” or running the command.

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