Create a Bootable macOS Sierra Installer on a USB Flash Drive

MacOS Sierra, 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.

The advantage of the ability to create a bootable installer of macOS Sierra can’t be overstated. It 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 can be a pretty nice feature if you have a problematic or slow connection to the Internet.

OS X and macOS have had the capability to create install media for quite a while, but this isn't widely known, for two reasons. First, the command to create the bootable installer is well hidden within the installer that's downloaded from the Mac App Store; and secondly, the installer you download has a really annoying habit of automatically starting up once the download is complete. If you then click the install button, you'll find that 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.

How to Create a Bootable Installer of macOS Sierra

USB Flash drive being inserted into a Mac Book Pro
Having the macOS Sierra installer on a bootable flash drive can be very convenient.

Before we begin the process of creating the bootable installer, you have a bit of 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 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 issues. In order to avoid any permanent problems, I highly recommend that you perform a backup of both your Mac and the media (USB flash drive or external drive) that you will be using. I cannot overstate the importance of performing these two tasks before you begin the installation process.

What You Need

  • A copy of the macOS Sierra installer, available from the Mac App Store. It sometimes takes Apple a little bit of time before the Mac App Store is updated with new releases such as macOS Sierra. If you don't see the operating system available in the store on day one, try performing a search for macOS Sierra in the store. The macOS Sierra installer will start up automatically once the download is completed. Do not continue with the installation as prompted by the installer; instead quit the installer.

If you allowed the installer to run, you'll need to re-download it.

Once it's downloaded, the installer can be found in the/Applications folder, with the name: ​Install macOS Sierra Public Beta. (This name will be updated as new versions are made available.)

  • A 16 GB or larger USB flash drive. We recommend a USB 3 version of the flash drive, to make the process of reading and writing data on the flash drive faster.

These instructions will also work for an external drive, however, for this guide, we'll assume that 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, where appropriate.

If you have everything, then let's get started.

Use Terminal to Create a Bootable macOS Sierra Installer

Terminal being used to create install media for macOS Sierra
Terminal can be used to create a bootable copy of macOS Sierra installer on a USB flash drive. Screen shot courtesy of Coyote Moon, Inc.

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.

The process we're going to use will completely erase the contents of the USB flash drive, so 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 from the Mac App Store. This command takes care of all the heavy lifting for you; it will erase and format the flash drive, then copy the macOS Sierra disk image that's stored within the installer to the flash drive. Finally, it will perform a bit of housekeeping magic, and mark the flash drive as bootable media.

The key to using the createinstallmedia command is the Terminal app. By using Terminal, we can invoke this command, sit back and take a short break, and then be presented with a bootable installer that we can use over and over again to install macOS Sierra on as many Macs as we wish.

Create the macOS Sierra Bootable Installer

Make sure the macOS Sierra installer file you downloaded from the Mac App Store is present in the /Applications folder on your Mac. If it's not there, you can jump back to earlier in this guide to learn how to re-download the installer.

Prepare the USB Flash Drive

  1. Connect the USB flash drive to your Mac.
  2. If the flash drive isn't already formatted for use with your Mac, you can 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 we'll be using in a moment. You can use any name you wish, but I’m going to make the following suggestions:
    • Don’t use any unusual characters; keep the name basic, just simple alphanumeric characters.
    • Don’t use any spaces in the name.
    • We highly recommend using the following name: macOSSierraInstall

That's the name we use in the command line example below. By using the same name, you can simply copy/paste the commands into Terminal, without having to make any modifications.

Create the Install Media

  1. With the flash drive connected to your Mac, launch Terminal, located in /Applications/Utilities.
  2. Warning: The following command will erase the contents of the flash drive. Make sure you have a backup of the drive, if needed, before continuing.
  3. In the Terminal window that opens, enter the following command. 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, remember the command is case sensitive. If you used a name for the flash drive other than macOSSierraInstall, you'll need to adjust the text in the command line to reflect the different name.
  4. The best way to enter the command is to triple-click the line below to 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.
    sudo /Applications/Install\ macOS\ --volume /Volumes/macOSSierraInstall --applicationpath /Applications/Install\ macOS\ --nointeraction
  5. Once you've entered the command into Terminal, press enter or return on your keyboard.
  6. You'll be asked for an administrator password. Enter the password, and press enter or return.
  7. The terminal will begin to execute the command and provide 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 is dependent on how fast the flash drive and interface is. Expect anywhere from a short wait to enough time for coffee and a snack.
  8. Once Terminal completes the task, it will display a line saying Done, and the normal Terminal command prompt will reappear.
  9. 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, you can 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.