You can start a Mac from a bootable USB installer by connecting the installer, opening the Mac’s startup options, and selecting the USB drive. The exact process depends on whether your Mac has Apple silicon—such as an M1, M2, M3, or later chip—or an Intel processor.
A bootable installer can help you install macOS, repair a startup problem, erase and reinstall macOS, or access installation tools without starting the system from its internal drive.
Quick Answer
- Connect the bootable macOS installer to your Mac.
- Shut down the Mac completely.
- Start it using the correct method:
– Apple silicon: Press and hold the power button until startup options appear.
– Intel: Turn on the Mac and immediately hold the Option (⌥) key. - Select the USB installer.
- Click Continue or press Return.
Before using the installer, save any important files. Installing or erasing macOS can delete data, and some repair actions may change the contents of the startup disk.
Before You Start
Make sure the USB drive is genuinely bootable and contains a compatible macOS installer. A regular USB drive containing a macOS application or copied files will not necessarily start a Mac.
If you still need to make the installer, follow our guide to create a bootable macOS installer.
You should also check the following:
- Back up important files. Use Time Machine or another backup method before reinstalling or erasing macOS.
- Connect the Mac to power. Do not allow the Mac to shut down during startup or installation.
- Use a working USB port or adapter. If the installer is not detected, try another port or a different adapter.
- Know the administrator password. macOS may request it when changing startup security settings or using recovery tools.
- Disconnect unnecessary accessories. External drives, hubs, and other devices can sometimes make startup troubleshooting more difficult.
Starting from the USB installer does not automatically erase your Mac. Data is at risk when you choose actions such as Erase in Disk Utility or erase the internal disk during macOS installation.
Start an Apple Silicon Mac from a USB Installer
This method applies to Macs with Apple silicon, including M-series MacBook Air, MacBook Pro, Mac mini, iMac, Mac Studio, and Mac Pro models.
1. Connect the USB installer
Insert the bootable USB installer directly into the Mac or connect it through a compatible adapter or hub.
If the Mac is already running, you can shut it down from the Apple menu:
- Select Apple menu.
- Click Shut Down.
- Wait until the screen is completely black and the Mac has stopped running.
Do not simply close the lid of a MacBook. Closing the lid puts the Mac to sleep rather than shutting it down completely.
2. Open startup options
Press and hold the Mac’s power button. On some MacBook models, the power button is also the Touch ID button.
Keep holding the button until you see Loading startup options or a screen showing available startup disks and a gear icon labeled Options.
You can release the button when the startup options screen appears.
3. Select the USB installer
Look for the USB drive in the list of available startup disks. It may appear with the installer’s name or the name assigned to the drive when it was created.
Select the USB installer and click Continue.
If the Mac asks you to choose a user, select an administrator account and enter its password. Depending on the Mac and installer, you may then see the macOS installer or the macOS Recovery utilities window.
4. Use the installer tools
The available options depend on the installer and the Mac. Common choices include:
- Install macOS: Installs macOS on a selected disk.
- Disk Utility: Checks, repairs, erases, or partitions storage.
- Restore from Time Machine: Restores a backup.
- Safari: May provide access to online help in Recovery.
- Startup Security Utility: Changes security settings on supported Macs.
Read each screen carefully before continuing. In particular, selecting Erase in Disk Utility permanently removes data from the selected volume or disk.
Start an Intel Mac from a USB Installer
This method applies to Macs with Intel processors. If you are unsure which processor your Mac has, select About This Mac from the Apple menu while macOS is running. Apple silicon Macs identify the chip as an Apple chip; Intel models show an Intel processor.
1. Connect the USB installer
Insert the bootable installer while the Mac is turned off. If the Mac is running, select Shut Down from the Apple menu and wait for it to turn off completely.
2. Open the Startup Manager
Press the power button to turn on the Mac. Immediately press and hold the Option (⌥) key.
Continue holding Option until the Startup Manager appears. This screen displays available startup disks, including external bootable drives.
If you are using a wireless keyboard and the key is not recognized, connect a wired keyboard if possible. A wired keyboard can also help when the Mac does not respond quickly enough to the startup key.
3. Select the USB installer
Use the arrow keys or mouse to select the USB installer. It may be labeled EFI Boot, Install macOS, or another name associated with the installer.
Press Return or click the on-screen arrow to start from the USB drive.
The Mac may take several minutes to load the installer. A progress bar, Apple logo, or blank screen during this stage can be normal. Do not disconnect the USB drive while the installer is loading.
4. Choose an installation or recovery tool
After startup completes, follow the onscreen instructions. You may see the macOS installer or a utilities window with options such as Disk Utility and Install macOS.
If you only need to reinstall macOS without erasing personal files, be careful to select the correct destination and avoid using Disk Utility to erase the internal drive unless you have a current backup and specifically intend to remove its contents.
If the USB Installer Does Not Appear
A missing USB installer usually means the Mac cannot recognize it as a valid startup disk, the drive is not connected correctly, or startup security settings are preventing it from being used.
Try these steps in order.
Check the connection
Disconnect and reconnect the USB installer. If the Mac has multiple ports, try another one. Avoid connecting through a monitor, keyboard, or complex USB hub during troubleshooting.
If the installer uses USB-C and the Mac uses a different connector, test another compatible adapter. Some adapters provide power or data but do not work reliably during startup.
Confirm that the installer is bootable
A drive containing the macOS installer application is not necessarily bootable. The installer must be created using a supported process and must have the required boot files.
If you created the drive yourself, recreate it on a different USB drive if possible. A failing or improperly formatted drive can appear in Finder but remain unavailable in Startup Manager.
Shut down completely and try again
Restarting is not always enough. Select Shut Down, wait until the Mac is off, connect the installer, and then use the correct startup procedure again.
For Apple silicon, hold the power button from a fully powered-off state. For Intel, press the power button and immediately hold Option.
Give Startup Manager time to load
External drives can take longer to appear than the internal startup disk. Keep the startup key held down on an Intel Mac until the disk-selection screen appears. On Apple silicon, continue holding the power button until startup options load.
Check Intel Mac startup security settings
Some Intel Macs with an Apple T2 Security Chip may restrict startup from external or removable media.
To review these settings:
- Start the Mac in macOS Recovery by turning it on and immediately holding Command (⌘)-R.
- Release the keys when the Apple logo or Recovery screen appears.
- From the menu bar, select Utilities > Startup Security Utility.
- Authenticate with an administrator account if asked.
- Review the settings for external or removable media.
Only change these settings if you understand the security consequences. Allowing startup from external media can make it easier for someone with physical access to start the Mac from another operating system or recovery environment. After you finish, consider restoring the strongest security setting that still supports your needs.
Check Apple silicon external startup policy
Apple silicon Macs use startup security policies managed through Recovery. If an external installer is blocked, start macOS Recovery by holding the power button until startup options appear, then select Options.
Depending on the Mac and macOS version, security-related settings may be available through the recovery environment or Startup Security Utility. If the installer is not accepted, verify that it is compatible with the Mac and that the Mac’s security policy allows the required startup method.
Reset the startup selection
If the Mac keeps starting from its internal disk, open Startup Manager again and manually select the USB installer. Choosing the installer once does not always change the Mac’s permanent startup disk.
To make a selected system the default on supported Macs, you can change the startup disk later in System Settings > General > Startup Disk on newer macOS versions, or System Preferences > Startup Disk on older versions.
If the Mac Starts Normally Instead
If macOS opens without showing the installer:
- Shut down the Mac instead of restarting it.
- Connect the USB installer before turning it on.
- Use the correct startup method for the Mac’s processor.
- On Intel models, press Option immediately after pressing the power button.
- On Apple silicon models, hold the power button until startup options appear.
- Try the installer in another USB port.
On an Intel Mac, a keyboard that connects too slowly can prevent the Option key from being recognized. Use a wired keyboard when available.
If the Installer Shows an Error
An error may indicate that the installer is damaged, incompatible, incomplete, or unable to verify the Mac’s startup environment.
Make sure:
- The Mac is connected to the internet if the installer requires verification or additional components.
- The Mac’s date and time are reasonably accurate when using Recovery tools.
- The installer is intended for the Mac model.
- The USB drive was created correctly and is not failing.
- You have enough available storage for the installation.
- You are installing to the intended volume.
If Disk Utility is needed, first select View > Show All Devices so you can distinguish the physical storage device from its volumes. Be extremely careful before selecting Erase. Erasing the wrong item can remove the operating system and personal files.
How to Stop Using the USB Installer
After completing the repair or installation, you can remove the USB installer when the Mac has finished starting from its internal drive.
If the installer is visible in Finder:
- Click the Eject icon next to the drive in the Finder sidebar, or drag it to the Trash.
- Wait for the drive to disappear.
- Disconnect it.
If the Mac is still running from the USB installer, do not remove it during installation or while files are being loaded. Choose Restart or Shut Down first, then remove the drive when it is safe.
Frequently Asked Questions
Does starting from a USB installer erase the Mac?
No. Selecting a USB installer only starts the Mac from that drive. Data is erased only if you choose an erase action, such as erasing a volume in Disk Utility or selecting an erase option during installation.
Which key starts an Intel Mac from USB?
Hold Option (⌥) immediately after turning on the Intel Mac. Keep holding it until Startup Manager appears, then select the USB installer.
How do I start an Apple silicon Mac from USB?
Shut down the Mac, connect the installer, and hold the power button until startup options appear. Select the USB installer and click Continue.
Can I use any USB drive?
The drive must contain a properly created, bootable macOS installer. A drive with copied installer files may not work. The Mac must also support the macOS version on the installer.
Why does the USB installer appear as “EFI Boot”?
On some Intel Macs, a bootable external drive may be labeled EFI Boot in Startup Manager. If it is the installer you created, select it and continue.
Can I install macOS without a backup?
You can, but it is risky. An installation can fail, and choosing the wrong erase option can permanently remove files. Create a current backup before making significant changes.
Final Thoughts
To start a Mac from a bootable USB installer, shut it down, connect the installer, and open startup options using the method for its processor. Hold the power button on Apple silicon Macs; hold Option on Intel Macs. Select the USB installer only after confirming that it is the correct drive, and back up important data before using installation or erase tools.