These advanced steps are primarily for system administrators and others who are familiar with the command line. You don't need a bootable installer to upgrade macOS or reinstall macOS, but it can be useful when you want to install on multiple computers without downloading the installer each time.

Jul 29, 2021 What you need to create a bootable installer. Create a bootable USB drive for macOS X versions including El Capitan, Yosemite, Mavericks, Sierra, High Sierra, Mojave, Catalina and Big Sur. Universal USB installer aka UUI is comparable to Rufus in respect to performance and compatibility. Stage 1: Create Bootable USB Flash Drive First connect your USB drive to the Mac computer and launch Disk Utility tool. Now select drive indicating USB flash drive (select the USB drive you want to make the bootable installer) from the available drives list.

What you need to create a bootable installer

• A USB flash drive or other secondary volume formatted as Mac OS Extended, with at least 14GB of available storage
• A downloaded installer for macOS Big Sur, Catalina, Mojave, High Sierra, or El Capitan

Download macOS

• Download: macOS Big Sur, macOS Catalina, macOS Mojave, or macOS High Sierra
  These download to your Applications folder as an app named Install macOS [version name]. If the installer opens after downloading, quit it without continuing installation. To get the correct installer, download from a Mac that is using macOS Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server.
• Download: OS X El Capitan
  This downloads as a disk image named InstallMacOSX.dmg. On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg. It installs an app named Install OS X El Capitan into your Applications folder. You will create the bootable installer from this app, not from the disk image or .pkg installer.

Use the 'createinstallmedia' command in Terminal

1. Connect the USB flash drive or other volume that you're using for the bootable installer.
2. Open Terminal, which is in the Utilities folder of your Applications folder.
3. Type or paste one of the following commands in Terminal. These assume that the installer is in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. If it has a different name, replace MyVolume in these commands with the name of your volume.

Big Sur:*

Catalina:*

Mojave:*

High Sierra:*

El Capitan:

* If your Mac is using macOS Sierra or earlier, include the --applicationpath argument and installer path, similar to the way this is done in the command for El Capitan.

After typing the command:

1. Press Return to enter the command.
2. When prompted, type your administrator password and press Return again. Terminal doesn't show any characters as you type your password.
3. When prompted, type Y to confirm that you want to erase the volume, then press Return. Terminal shows the progress as the volume is erased.
4. After the volume is erased, you may see an alert that Terminal would like to access files on a removable volume. Click OK to allow the copy to proceed.
5. When Terminal says that it's done, the volume will have the same name as the installer you downloaded, such as Install macOS Big Sur. You can now quit Terminal and eject the volume.
   

Use the bootable installer

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

1. Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.
2. Turn on your Mac and continue to hold the power button until you see the startup options window, which shows your bootable volumes.
3. Select the volume containing the bootable installer, then click Continue.
4. When the macOS installer opens, follow the onscreen instructions.

Intel processor

1. Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.
2. Press and hold the Option (Alt) ⌥ key immediately after turning on or restarting your Mac.
3. Release the Option key when you see a dark screen showing your bootable volumes.
4. Select the volume containing the bootable installer. Then click the up arrow or press Return.
   If you can't start up from the bootable installer, make sure that the External Boot setting in Startup Security Utility is set to allow booting from external media.
5. Choose your language, if prompted.
6. Select Install macOS (or Install OS X) from the Utilities window, then click Continue and follow the onscreen instructions.

Learn more

A bootable installer doesn't download macOS from the internet, but it does require an internet connection to get firmware and other information specific to the Mac model.

For information about the createinstallmedia command and the arguments you can use with it, make sure that the macOS installer is in your Applications folder, then enter the appropriate path in Terminal:

Update: Go to Install macOS Sierra Using Bootable USB Flash Drive if you want to install macOS 10.12 Sierra instead.

Update 2: The instructions below will work to manually create a Mac OS X 10.11 El Capitan USB installation drive. I needed to upgrade my sister’s 2008 Macbook Pro and found that the existing Mac OS X 10.6 Snow Leopard did not support the “createinstallmedia” tool; I got a “Failed to start erase of disk due to error (-9999, 0)” error. I followed the instructions below to successfully create a bootable installer for Mac OS X 10.11 El Capitan, which is the last version to support the 2008 Core 2 Duo Macbook Pro.

In this post, I will go over instructions on how to create a bootable USB flash drive containing the Mac OS X 10.10 Yosemite installer. These instructions will also work for Mac OS X 10.9 Mavericks (excluding a Yosemite-specific step) and differ significantly from the instructions for creating a Mac OS X 10.6 Snow Leopard installer. You will need an 8GB USB flash drive for Mac OS X Yosemite or Mavericks.

I tried several methods which failed to create a bootable USB flash drive before finding one that succeeded. The instructions I found that worked, using Disk Utility, were located at How to Make a Bootable OS X Mavericks USB Install Drive and How to Create a Bootable Install USB Drive of Mac OS X 10.10 Yosemite.

Download the Mac OS X 10.10 Yosemite

First, download the latest Mac OS X version, which is 10.10 Yosemite. It is the version currently available for download from the “App Store”. (If you want an earlier version like Mac OS X 10.9 Mavericks, you’ll need to get it from elsewhere.)

Launch “App Store” and search for “OS X Yosemite”. Download it. (It is 5.16GB in size.)

Note: If you run the Yosemite installer to upgrade your Mac, the downloaded installer file will be deleted automatically after the upgrade is completed. To keep that file, you will want to move it out of the Applications folder so it won’t be deleted after an upgrade. Launch the “Terminal” app and run this command to move the downloaded installer app to your user’s “Downloads” folder:

Create Bootable USB Flash Drive Installer

By default, the Finder will hide system files which we will need to see. Run these commands in the “Terminal” app to expose the hidden files:

Prepare the USB flash drive:

Yosemite Install Usb Stick

1. Plug in a USB flash drive of size 8GB or larger.
2. Launch the “Disk Utility” to format the USB Flash drive.
3. On the left-hand pane, select the USB drive (not the partition under it, if any).
4. Click on the “Erase” tab, select “Mac OS Extended (Journaled)” for “Format” and input a name like “Install Yosemite” (or anything because this name will be overwritten later).
5. Click the “Erase…” button at the bottom and then the “Erase” button in the popup dialog. This format operation should take less than a minute to complete.

Restore the Yosemite installation image to the USB flash drive:

1. Launch the Finder and locate the “Install OS Yosemite.app” file. Right-click (hold the “control” key and click) on it and select “Show Package Contents”.
2. Open Contents, then SharedSupport, and double-click on the InstallESD.dmg (disk image) file to mount it. A volume called “OS X Install ESD” will show up on the desktop and under DEVICES in the Finder.
3. In the “OS X Install ESD” volume, right-click on the “BaseSystem.dmg” file and select “Open” to mount it. (Double-click won’t perform any action because it is a hidden file.)
4. Use Disk Utility to clone the “BaseSystem.dmg” to the USB flash drive:
   1. Select the “BaseSystem.dmg” in the left-hand pane and click on the “Restore” tab. The “Source” field will be populated with “BaseSystem.dmg”.
   2. Drag the “Install Yosemite” partition under the USB flash drive to the “Destination” field.
   3. Click the Restore button and then the Erase button.
   4. The USB flash drive will be written with the contents of “BaseSystem.dmg” file. Depending on the speed of your USB flash drive, it may take several minutes or longer to complete this operation.
   5. Once complete, the “Install Yosemite” partition will be renamed to “OS X Base System”.
5. Use the Finder to navigate to the USB flash drive. You will see two “OS X Base System” volumes in the Finder’s left-hand pane. The USB flash drive is the last one.
6. Under the USB flash drive’s “OS X Base System” partition, open the “System/Installation” folder. You will see an alias file named “Packages”. Delete it because we will replace it with a “Packages” folder below.
7. Use a second Finder window to open the “OS X Install ESD” volume. (To open a second Finder window, you can use the Finder menu’s “File/New Finder Window” command.)
8. Copy the “Packages” folder from the “OS X Install ESD” volume to the USB flash drive’s “System/Installation” folder.
   
9. Required for Yosemite (not required for Mavericks): Copy the “BaseSystem.chunklist” and “BaseSystem.dmg” files from the “OS X Install ESD” volume to the USB flash drive’s root “/” folder. If you don’t do this, you will get an “undefined error 0” when attempting to install Yosemite.
10. The USB flash drive is now complete. You can use it to boot a Mac to install Mac OS X 10.10 Yosemite.
11. Unmount all the Yosemite installer volumes by ejecting them; you must eject “OS X Base System” before “OS X Install ESD”.

Re-configure the Finder to hide system files. Run these commands in the “Terminal” app:

Boot With USB Flash Drive

Yosemite Os Installer

To boot a Mac with the USB flash drive:

1. Insert the USB flash drive.
2. While holding the “option/alt” key down, turn on the Mac to display the boot Startup Manager.
3. You should see one or two icons, one for the internal hard drive and/or another called “OS X Base System” for the USB flash drive. (The internal hard drive may not be visible if it does not have a bootable partition installed.)
   • Note: If you don’t see the USB flash drive’s “OS X Base System”, try removing and re-inserting the USB flash drive while viewing the Startup Manager screen. The USB flash drive should then appear after a few seconds.
4. Select the “OS X Base System” and hit the “return/enter” key to boot from the USB flash drive.

Create Bootable Usb Yosemite Installer

Hopefully, this post will help you to create your own bootable USB flash drive installer for Mac OS X 10.10 Yosemite or Mac OS X 10.9 Mavericks.