The first thing you need is a flash drive, of course. For macOS High Sierra, Apple recommends that you use a USB drive with at least 12GB of storage. It is also better to use a USB 3.0 flash drive, as both the process of creating the installer USB and installing macOS from it will be faster that way.
Preparing the USB drive
Once you have your USB drive, you need to wipe it, which can be done with just a few steps.
- Open Disk Utility
- Select your USB drive from the sidebar
- Click Erase
- Enter a catch name, something like "macOS Installer"
- Select Mac OS Extended (Journaled) as the file system, and choose GUID Partition Map if asked.
Next, you need to download a macOS installer from the Mac App Store. You can either just download the latest version of macOS High Sierra, or, if you've downloaded older versions before, find previous versions of macOS, which would be listed and available for download under the 'Purchased' tab in the Mac App Store.
With that, you now have all the pieces required for creating a macOS installer USB.
Creating the Installer USB drive
To copy the installation files to the USB drive and to make it bootable, first, you will need to copy the command that you will use, which will be different depending on your macOS version.
For macOS High Sierra:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/macOS\ Installer
For macOS Sierra:
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/macOS\ Installer --applicationpath /Applications/Install\ macOS\ Sierra.app
For OS X El Capitan:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/macOS\ Installer --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
For OS X Yosemite and installers for other older versions of the operating system, use the same command as for OS X El Capitan, but replace all instances of
Install\ OS\ X\ El\ Capitan with
Install\ OS\ X\ Yosemite, or whatever the name of the OS X installer application is.
Remember that all spaces in the file name and the path (including the name of the USB drive's volume) need to have a backslash before them, like this:
\. This is important because otherwise, the
createinstallmedia application will interpret everything after the space character as a new command. Also, remember to rename
macOS\ Installer to whatever the actual name of your USB volume is.
Once you have your installer command:
- Open the Terminal
- Paste your command
- Enter your password
- Confirm that you are okay with all the data on the flash drive being erased
After that, the process will begin. It might take some time depending on the macOS version and the USB drive that you are using, but once it is complete, your new installer USB will be ready.
Then, just reboot the computer you want to reinstall or upgrade macOS on, hold the
⌥) key, select the drive you just created, and follow the installation steps.