Creating a Bootable macOS Installer Drive: Step-by-Step Guide

Introduction / Why You Need This

Creating a bootable macOS installer disk involves preparing a USB drive that contains a full copy of the operating system. Such media is indispensable in several scenarios:

• Clean macOS installation: Install the system from scratch on a new or formatted disk without needing to download gigabytes of data via the internet.
• System recovery: Boot your Mac into Recovery Mode from the flash drive if the built-in recovery partition is damaged or inaccessible.
• Updating multiple computers: Install the same version of macOS on several Macs on a network without internet access.
• Creating a bootloader backup: In case of problems booting from the internal disk.

After completing this guide, you will have a portable tool for deploying macOS.

Requirements / Preparation

Before you begin, ensure the following conditions are met:

1. Compatible Mac: You must have a working Mac (even with a problematic system) where you can run the App Store or Terminal.
2. macOS installation image: Download the required version of macOS from the App Store. This is typically an Install macOS [Name].app file in the /Applications folder. If the image already exists on disk, ensure it is not corrupted.
3. USB drive:
   • Capacity: at least 16 GB (for macOS Ventura and newer). For older versions (Catalina, Mojave), 12 GB is sufficient.
   • Type: USB 3.0 or higher for speed, but USB 2.0 also works.
   • Important: All data on the flash drive will be permanently erased.
4. Administrator privileges: The createinstallmedia command and disk formatting require superuser (sudo) rights.
5. Stable internet connection: To download the installation image (if not already downloaded). The image size is 10-15 GB.

Step-by-Step Instructions

Step 1: Identifying the USB Drive Identifier

Connect the prepared flash drive. Open Terminal (Applications → Utilities → Terminal) and run:

diskutil list

In the output, find your drive. It usually has a name like UNTITLED or MYUSB, and a path like /dev/disk2s1. You will need only the main disk identifier (without the partition number), for example /dev/disk2. Remember it.

⚠️ Important: Be extremely careful! Make sure you select the external USB drive and not your Mac's internal SSD. Formatting the internal drive will result in complete data loss.

Step 2: Formatting the USB Drive (if necessary)

If the flash drive has an old file system (e.g., FAT32 or exFAT) or you want to start fresh, format it in Mac OS Extended (Journaled) or APFS (recommended for macOS Catalina and newer).

In Terminal, run (replace diskX with your identifier from step 1):

sudo diskutil eraseDisk APFS "Untitled" /dev/diskX

• APFS — the file system. For compatibility with older Macs, you can use JHFS+ (Mac OS Extended (Journaled)).
• "Untitled" — the volume name that will appear on the desktop. You can set your own.
• /dev/diskX — your disk.

💡 Tip: After formatting, the disk will automatically mount and appear in Finder with the name Untitled (or the one you set).

Step 3: Running the createinstallmedia Command

This is the key step. Assuming you downloaded macOS Sonoma, the installer app will be located at /Applications/Install macOS Sonoma.app.

In Terminal, run (replace Sonoma with your version's name, and Untitled with your mounted USB disk's name):

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

What the command does:

• sudo — runs with administrator privileges.
• Path to createinstallmedia — this is the internal script inside the installer app.
• --volume /Volumes/Untitled — specifies which mounted volume to write the image to.

During the process, you will see progress as a percentage. Do not interrupt the process or remove the flash drive! Creating the installer media will take 10 to 30 minutes depending on USB speed.

If you have a different version of macOS, replace Sonoma in the path with the current name:

• Install macOS Sequoia.app
• Install macOS Ventura.app
• Install macOS Monterey.app
• Install macOS Big Sur.app
• Install macOS Catalina.app

Alternative: If you are unsure of the exact folder name, drag the Install macOS [Name].app folder from Finder directly into the Terminal window. The path will be inserted automatically.

Step 4: Completion and Verification

Upon completion, Terminal will display the message Install media now available at "/Volumes/Install macOS Sonoma". This means that a bootable installer has been created on your flash drive.

In Finder, you will see that the flash drive has been renamed (for example, to Install macOS Sonoma). Do not format or modify the contents of this volume!

Verifying the Result

1. Visual inspection: In Finder, open the external disk. You should see files like Install macOS Sonoma.app (or similar), com.apple.recovery.boot, BaseSystem.dmg, and other system files. The total used space should match the size of the installation image (10-15 GB).
2. Booting from the flash drive (recommended method):
   • Disconnect and reconnect the flash drive.
   • Restart your Mac while holding the Option (⌥) key.
   • In the boot menu, you should see the disk named Install macOS Sonoma (or your version). Select it and press Enter.
   • The macOS Installer (installation assistant) should launch. This confirms that the media is working.
3. Verification via diskutil: In Terminal, run diskutil list and find the volume with the installer name. Its type should be Apple_FS or Apple_HFS.

Potential Issues

Error "Could not create a preboot volume" or "Failed to update the preboot volume"

Cause: This usually occurs due to incompatibility between the createinstallmedia version and the disk format (for example, attempting to write to a disk in FAT32 format) or permission issues. Solution: Fully format the disk in APFS (or Mac OS Extended) via Disk Utility or the diskutil eraseDisk command, as shown in step 2. Ensure you are using the latest version of the installer app.

Error "The disk you selected was not large enough"

Cause: The flash drive does not have enough free space for the image. Solution: Use a flash drive with at least 16 GB of capacity. Check that you are not trying to write the image to an already full partition. Reformat the entire disk, not just a partition.

createinstallmedia command not found

Cause: The macOS installer app is not downloaded or is not in the standard /Applications folder. Solution: Download macOS from the App Store. Ensure the app file is located in /Applications, not in the Downloads folder. Check the path by dragging the app into Terminal.

Mac does not see the flash drive when booting with Option (⌥)

Cause:

1. The flash drive was created incorrectly.
2. On a Mac with an Apple Silicon chip (M1/M2/M3), booting from external media may require connecting the flash drive directly to the Mac's port, not through a hub.
3. On older Intel-based Macs, resetting NVRAM/PRAM (Cmd+Option+P+R at boot) sometimes helps. Solution: Recreate the media by following the instructions. Ensure you are using a macOS version compatible with your Mac (an image for a newer version may not boot on older hardware).

Process is stuck or taking too long

Cause: Slow USB port (USB 2.0), flash drive with low write speed, or corrupted image. Solution: Use a high-quality USB 3.0 flash drive or external SSD. Check the integrity of the downloaded image (its size in the Applications folder should match what is stated on the App Store page). If the problem persists, try a different flash drive.