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A Step-By-Step Guide to Boot Your Mac With a USB Drive

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You might be wondering why you would want to boot macOS from a USB drive. Usually, if you needed to upgrade your Mac’s operating system, you’d perform a standard macOS upgrade via the App Store. But what if you need to upgrade multiple Macs? It’s much faster to use a bootable drive instead of going through the App Store process – which includes searching for the upgrade, entering usernames and passwords, and running an installer – on each Mac.

There are also other reasons why you might want to boot your Mac from a USB drive. Here are a couple of the most common: 

  • Performing a clean install of macOS
  • Accessing a Mac that won't start up
  • Testing out a beta version of macOS
  • Downgrading to an earlier operating system
  • Being prepared for unexpected situations with an emergency disk
  • Upgrading macOS on multiple Macs

Now that you know why you might want to boot from USB, here’s how to do it. Basically, there are three steps to follow: 

  1. Create an installer drive
  2. Download macOS and install it onto your external drive
  3. Boot up your Mac with the drive plugged into it

 

Creating an installer drive: Prerequisites and steps to follow

Prerequisites:

  • Before getting started with the installer disk creation, don’t forget to back up your Mac.

  • The installer software will take up over 12GB, so make sure you have enough space on your USB device.  

  • You need to format your USB device for your Mac. Start with a clean thumb drive or hard drive to make it a bootable drive.

  • Proceed only if you are logged in as an administrator and you know the admin password.

Note: To connect a USB device with a type-A connector to a MacBook or MacBook Pro with USB-C, you need the USB to USB-C adapter. For a 2015 or newer MacBook or a 2016 or newer MacBook Pro, you need Apple’s USB to USB-C adapter. This will allow you to connect a storage device that uses a USB type-A connector. If you have a USB-C storage device, then you don’t need to get the adapter.

Steps to Follow:

  • Plug the thumb drive or cable for your hard drive into your Mac’s port.

  • Open Finder and choose Applications.

  • Scroll down and double-click on Utilities.

  • Find and double-click on Disk Utility.

  • Select your drive under External.

  • Click on the Erase tab at the top of the window.

  • Note: remember the name of your external hard drive (by default, it’s "Untitled"), you will need it when you create a bootable drive. If there’re more than one external drive with the same name, rename the drive you are using as a bootable installer.

  • Choose Mac OS Extended (Journaled) from the format list and click Erase.

  • Once the process is finished, click Done and quit Disk Utility.

Downloading and installing macOS: Prerequisites and steps to follow

Prerequisites:

  • Download macOS only from the App Store or via Apple’s support page. Ensure that you choose either the Mac operating system that came with your Mac or a compatible newer version.

  • This guide requires the use of Terminal. If you don't feel comfortable making changes to your Mac with Terminal, you can create a bootable disk using special apps, for instance, DiskMaker X or Install Disk Creator.

Steps to Follow

  • Download the required version of the Mac operating system from the App Store.

  • When the macOS installer opens, quit it without continuing installation.

  • Open Finder and choose Applications.

  • Find the installer as a single "Install" file.

  • Connect the USB device or other volume that will be used as the bootable installer.

  • Launch Terminal by entering Terminal in Spotlight.

  • Copy and paste one of the following commands in Terminal (depending on the the Mac operating system you want) and press Enter.

    for High Sierra:

    sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ High\ Sierra.app

    for Sierra:

    sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app

  • When prompted, enter your admin password and press Enter again. Note: Terminal won’t show any characters as you type your password.

  • Type Y to confirm that you want to erase the volume, then press Enter. Terminal shows the progress as the bootable installer is created.  

  • Once finished, the volume will have the same name as the installer you downloaded, such as Install macOS High Sierra.

  • Quit Terminal and safely eject the volume.

Booting from USB: Steps to follow

  • Turn off your Mac.

  • Connect the drive to your Mac via the USB port.

  • Turn on your Mac.

  • Hold down the Alt key when your Mac starts up.

  • Select the external drive with the Mac operating system on it from the list of systems to start up your computer. (You can use either Startup Disk preferences or Startup Manager)

Use Startup Disk preferences

By default, if you use Startup Disk preferences to select a startup disk, your computer will start up from that disk until a different one is specified.

  1. Go to the Apple menu, choose System Preferences, then click Startup Disk.

  2. Click the lock and enter your admin password.

  3. Select your startup disk, then click Restart.

If there’s a message saying that your security settings don’t allow your computer to use an external startup disk, check the External Boot setting in the Startup Security Utility.

Use Mac Startup Manager

If you use Startup Manager to select a startup disk, your computer will start up from that disk only once. Afterwards, it will use the disk that is specified in Startup Disk preferences.

  • Press and hold the Alt key as you restart your Mac.

  • Once the Startup Manager window appears, release the Alt key.

  • If your Mac is protected by a firmware password, you can release the key when you're asked to enter the password.

  • Select your startup disk, then click the arrow under its icon, or hit Enter.

Tip: Press and hold the Ctrl key during this step and your selection will be saved in your Startup Disk preferences until you change it.

Important: Ensure the drive is properly plugged in, because if it is not, the drive will most likely fail to mount. If your drive is powered via a USB cable, ensure that adequate power is being delivered to the drive. Older Macs may require a USB power cable, a cable that splits into two USB connectors that need to both be plugged into your Mac, in order to deliver enough power to the drive. Similarly, make sure that the drive doesn't have an external power supply that it should be using.

There's always a moment of panic when your Mac refuses to boot. But don't worry too much – here are some useful strategies to get your Mac to boot:

  • Recovery Mode: Press and hold down Cmd and R keys simultaneously until you see a window with Mac OS Utilities on your screen. Follow the instructions.

  • Verbose Mode: Press and hold Cmd and V till you hear the startup tone. After that, you'll see rows of text scroll onto your screen, and you may be able to identify the problem.

  • Startup Manager: Reboot your Mac, then press and hold down Alt and C or N:
    a) C during the startup to boot directly from an inserted CD/DVD or USB bootable drive.
    b) N to start your Mac from a network-based operating system.

So, what if you mac has succesfully launched but you don't see the USB drive? Don't panic. You can solve this. And just for the record, this situation often occurs if you initially eject the external drive and unplug it from your computer.

How to solve:

Method 1: Ensure your Mac is set to show mounted drives on the desktop. Open Finder, click Finder in the upper menu, choose Preferences and then General and make sure that there is a tick beside External disks.

Method 2: Try a Super Slow Jam! Surprisingly, some users found that their thumb drives work when they plug it in extremely slowly. You should try this as well.

Method 3: Completely remove the external drive and then restart your Mac several times until the drive finally shows up again.

The Bottom Line

Booting your Mac from an external USB drive is not difficult, but it requires some time and skill. If you just want to upgrade your macOS, it’s easier to go your Mac’s App Store and find the update and simply follow the recommendations. But there are situations when you might need to boot your Mac from an external USB drive and you should not be afraid of doing this – just make sure you follow the instructions.

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