Create a bootable macOS USB installer

You do not need to find a macOS installer before you start. Install Disk Creator has a built-in downloader: choose a macOS release, download it directly from Apple, then use it to create your bootable USB. You can also select an installer you already have.

A USB installer is useful for installing macOS on several compatible Macs, keeping installation media handy, or trying another installation method when an upgrade fails. For an ordinary upgrade, you can use Software Update. You do not need to erase your Mac just because a new macOS version has arrived.

1. Choose a macOS version your Mac supports

Check the Mac you want to install macOS on, as well as the Mac you use to prepare the USB. Install Disk Creator runs on macOS High Sierra 10.13 or later and is a universal app for Apple silicon and Intel. That does not make every macOS installer compatible with every Mac.

• macOS 27 Golden Gate: requires a supported Apple silicon Mac. Check Apple’s current compatibility list. Choose the public release in Install Disk Creator when available, or select a full installer obtained from Apple.
• macOS 26 Tahoe: supports Apple silicon and selected Intel models. Check Apple’s Tahoe compatibility list before choosing it for an Intel Mac.
• Sequoia and older: choose a release supported by the destination Mac. The downloader can show releases for other Macs: turn off Compatible Only to browse them. Downloading a release does not guarantee that your current Mac can assemble its installer or that the destination Mac can boot it.

The downloader’s Compatible Only filter refers to the Mac running the app. If you are preparing a USB for another Mac, check that destination Mac’s requirements separately. The list of available downloads comes from Apple’s software catalogs; use Refresh to fetch the list again.

2. Prepare your USB and backup

• A 32GB or larger USB drive, or an external SSD. Smaller drives can fit some older releases; 32GB is the recommended starting point for current installers.
• Space on your Mac for the full installer. Check the download size and allow room for it to unpack.
• An internet connection. You need it to download macOS, and the destination Mac needs it during installation for firmware and model-specific information.
• Administrator access on the Mac creating the installer.
• A separate backup of any files you need to keep.

Before upgrading or reinstalling, make a current backup and check that you can read it. You can use Time Machine, or Mac Backup Guru for file copies and incremental snapshots. Decide how you will restore your files before erasing a Mac; a USB installer is not a backup of your data.

3. Download macOS and create the USB

Get macOS with the built-in downloader

1. Download and open Install Disk Creator. Open the downloaded ZIP and move the app to Applications.
2. Click Download macOS Installer… in the main window. The downloader loads the available macOS installers from Apple.
3. Find the release you want. Search by macOS name, version or build. Leave Include Betas off for public releases. Compatible Only narrows the list to the current Mac; turn it off to browse installers for other Macs.
4. Click Download beside the chosen release. You can use Pause and Resume while downloading. Allow space for both the downloaded packages and the resulting installer app.
5. Follow the completion prompt. If offered, click Assemble Installer and authorize it to create the installer app in Applications. If the app instead reveals a downloaded package, follow its instructions to open InstallAssistant.pkg and place the installer app in Applications. This prepares the installer app; it does not upgrade your Mac.

If the macOS installation window opens, quit it. Return to Install Disk Creator and select the resulting Install macOS … app. Already have a full installer? Skip the download and select that app in the main window.

Write the installer to your USB

1. Connect your USB directly and select it in Install Disk Creator. Check its name and capacity. The selected volume will be erased, so keep your backup on a separate disk.
2. Check the selected macOS installer, then click Create Installer. Approve the requested administrator authorization. Install Disk Creator uses Apple’s createinstallmedia tool to write the USB.
3. Wait for completion, then eject the USB. Keep the drive connected while it is being written. Downloading macOS, preparing the installer app and writing the USB are separate stages; timing depends on your connection, installer size and drive speed.

4. Start the destination Mac from the USB

Shut down the Mac, connect the completed installer directly, and make sure an internet connection is available. The startup procedure depends on the Mac’s processor.

Apple silicon Macs

1. Press and hold the power button until startup options appear.
2. Select the USB installer and click Continue.
3. Follow the macOS installer’s instructions.

Intel Macs

1. Turn on the Mac and immediately hold Option (Alt).
2. Release the key when the startup volumes appear.
3. Select the USB installer, then press Return or click the arrow.
4. Follow the onscreen instructions to install the compatible macOS version.

If an Intel Mac with a T2 chip refuses to start from the USB, check the external-media setting in Startup Security Utility. Apple’s instructions explain when to allow booting from external or removable media.

Reinstalling and erasing are separate choices

Starting from a USB does not require erasing your Mac. Reinstalling onto an existing compatible macOS volume can preserve your files, but you should still have a backup. If you deliberately want a clean install, follow Apple’s erase instructions for your exact Mac and confirm your recovery plan first. Copying an old home folder back is not a general substitute for a supported migration or restore procedure.

Troubleshoot common problems

The macOS version I want is not in the download list

Click Refresh, clear the search field and check the filters. Turn off Compatible Only if you are looking for an installer for another Mac. Turn on Include Betas only if you want a beta release. If the installer is still unavailable, check Apple’s download options.

The installer is not recognised

Make sure the downloader has finished preparing the installer app. If you stopped after downloading the packages, use the completion prompt’s assembly instructions or open the downloaded InstallAssistant.pkg as directed. Then select the full installer app from Applications. A package or incomplete download cannot be written to USB as an installer app.

The USB is too small or cannot be erased

Try a 32GB or larger drive and connect it directly. Check that no files on the target are open. If erasing still fails, consult Apple’s bootable-installer troubleshooting before reformatting; confirm the disk selection before any erase operation.

The USB does not appear at startup

Confirm that creation finished, reconnect the drive directly, and use the startup steps for your processor above. On an Intel Mac with T2, check external-boot permission. A different cable or port may help if the drive is not detected.

The Mac shows a prohibited symbol or will not install

Check that the macOS version supports the destination Mac. A bootable USB cannot bypass Apple’s hardware requirements. Keep the Mac connected to the internet so the installer can obtain required firmware and other information.

Still stuck? Contact MacDaddy with your Mac model, current macOS version, selected installer version, drive capacity and the exact error message.

Prefer to create the installer with Terminal?

Apple documents the manual createinstallmedia procedure, including commands for the releases covered by its guide. Use the command matching your full installer app and substitute the correct USB volume name. The command erases the target volume.

Follow Apple’s bootable-installer instructions, or use Install Disk Creator to choose the installer and drive through a Mac interface.

References: Apple’s bootable-installer guide, macOS downloads and Startup Security Utility.