Create a Bootable macOS Sierra Installer on a USB Flash Drive

Create a bootable version of macOS Sierra

macOS Sierra (10.12), the first of the new macOS systems, includes the ability to create a bootable installer on a USB flash drive or on a drive you have connected to your Mac.

A bootable installer of macOS Sierra allows you to perform a clean install, which completely replaces the contents of your Mac’s startup drive with a brand-new, fresh install of Sierra. The bootable installer can also be used to install macOS Sierra on multiple Macs, without having to resort to downloading the installer app from the Mac App Store each time. This is a nice feature if you have a problematic or slow connection to the internet.

Installation media for OS X and macOS has been available for some time but is not often used. The command to create the bootable installer is hidden in the installer that you download from the Mac App Store. That installer download automatically starts up when the download is complete. If you click the install button, the installer you downloaded is automatically deleted as part of the normal installation process, preventing you from using it to create a bootable macOS Sierra installer of your own.

USB Flash drive being inserted into a Mac Book Pro

How to Create a Bootable Installer of macOS Sierra

Before beginning the process of creating a bootable installer, you may have some housekeeping to perform. Creating the bootable installer requires that the bootable media (a flash drive or external drive) be formatted, resulting in the erasure of any data that the target volume may contain.

In addition, the commands to create the bootable installer require the use of Terminal, where a wrongly entered command can cause unexpected problems. In order to avoid any permanent problems, perform a backup of both your Mac and the media (USB flash drive or external drive) that you will be using.

What You Need

The macOS Sierra installer will start automatically when the download is complete. Do not continue with the installation as prompted by the installer. Instead, quit the installer. If you allow the installer to run, you will need to re-download it.

  • 16 GB or larger USB flash drive: A USB 3.0 flash drive makes the process of reading and writing data on the drive fast. These instructions also work for an external drive, but this guide assumes you're using a USB flash drive. If you're using an external drive, you should be able to adapt the instructions for your needs.

Use Terminal to Create a Bootable macOS Sierra Installer

With a copy of the macOS Sierra installer downloaded from the Mac App Store and a USB flash drive in hand, you're ready to begin the process of creating a bootable macOS Sierra installer.

Terminal being used to create install media for macOS Sierra

Coyote Moon, Inc.

This process completely erases the contents of the USB flash drive. Make sure you have the data on the flash drive backed up or that you don't care about the loss of any data it may contain.

The createinstallmedia Command

The key to creating the bootable installer is the use of the "createinstallmedia" command that is tucked away inside the macOS Sierra installer you downloaded. This command takes care of all the heavy lifting for you: It erases and formats the flash drive and then copies the macOS Sierra disk image that's stored in the installer to the flash drive. Finally, it performs a bit of magic and marks the flash drive as bootable media.

The key to using the "createinstallmedia" command is the Terminal app. By using Terminal and invoking this command, you can sit back, relax, and be presented with a bootable installer that you can use repeatedly to install macOS Sierra on as many Macs as you want.

Create the macOS Sierra Bootable Installer

Here's how to create a macOS Sierra bootable installer using a USB flash drive.

Prepare the USB Flash Drive

First, you need to prepare the flash drive for the macOS Sierra installer.

  1. Connect the USB flash drive to your Mac.

  2. If the flash drive isn't formatted for use with your Mac, use Disk Utility to format the flash drive using one of the following guides:

  3. The flash drive needs to have a unique name for use in the "createinstallmedia" command. You can use any name you want, but select a basic name with simple alphanumeric characters—no unusual characters. This example uses the the name "macOSSierraInstall."

Create the Install Media

The final but most complex task is to create the install media.

The following command erases the contents of the flash drive. Make sure you have a backup of the drive, if needed, before continuing.

  1. With the flash drive connected to your Mac, launch Terminal, located in /Applications/Utilities/.

  2. In the Terminal window that opens, enter the following command.

    sudo /Applications/Install\ macOS\ --volume /Volumes/macOSSierraInstall --applicationpath /Applications/Install\ macOS\ --nointeraction

    The best way to enter the command is to triple-click select the entire command, copy (Command+C) the text to your clipboard and then paste (Command+V) the text into Terminal, next to the command prompt.

    The command is a single line of text, though it may appear as multiple lines in your browser. If you type the command into Terminal, the command is case sensitive. If you used a name for the flash drive other than macOSSierraInstall, adjust the text in the command line to reflect the different name.

  3. After you enter the command into Terminal, press Enter or Return on your keyboard.

  4. You'll be asked for an administrator password. Enter the password and then press Enter or Return.

  5. The terminal begins to execute the command and provides you with status updates as the process unfolds. Most of the time is spent writing the installer image to the flash drive. The time it takes depends on how fast the flash drive and interface are.

  6. Once Terminal completes the task, it displays a line saying Done, and the normal Terminal command prompt reappears. You can now quit Terminal.

The bootable USB flash drive for installing macOS Sierra has been created. Be sure to properly eject the drive if you plan to use it on a different Mac or keep it connected to your Mac to start a clean install of macOS Sierra.

The bootable installer contains a number of utilities, including Disk Utility and Terminal, that you can use for troubleshooting your Mac if you ever have startup problems.

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