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.

What you need to create a bootable installer

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

Mar 04, 2021 How to Perform a Clean Install of OS X El Capitan. OS X El Capitan (OS X 10.11) offers two installation methods. This guide focuses on the 'clean install' method. When you install El Capitan on your current startup drive with the clean install method, you erase everything on the drive. That includes OS X, your user data, and personal files. I created a USB disk with a bootable El Capitan as per this documentation. Plug-in your USB key on a turned-off computer. Start your Mac, press immediately OPTION (aka ALT) Select the Orange icon (aka a USB stick) labelled El Capitan. If it does not detect it after 10 sec and you only see your hard-drive. Try another USB port and wait 10 sec again. It installs an app named Install Version Name. Open that app from your Applications folder to begin installing the operating system. Once you have the El Capitan installer extracted from the package pkg: How to create a bootable installer for macOS - Apple Support. You must use Safari to start the kit download.

Download macOS

• Download: macOS Big Sur, macOS Catalina, macOS Mojave or macOS High Sierra
  These will be downloaded 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 will be downloaded 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 will install 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 has been erased, you may see an alert stating 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 been completed, 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 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 the macOS installer is in your Applications folder, then enter the appropriate path in Terminal:

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What is the Model of your Mac?:

Note that an Apple computer cannot install a macOS that was releases prior to the date of manufacturing. So, if El Capitan was released prior to this model, then this is not an option. So, what is the year of this MacBook Pro? To find out: Go to: Apple menu > Select: About This Mac > View: Second Line-Down.

What is your macOS Version?

If you need to get an older macOS version, refer to this link:How to Get Old Versions of macOS - Apple Support.

Reinstalling the macOS:

Go Here: How to reinstall macOS from macOS Recovery - Apple Support. Important: Be certain to create a backup of your Mac prior to all of this. That way, you can have something to restore from the Mac from, should anything go wrong with the reinstall. Once reinstalled, try this once more.

El Capitan Reinstall

Sep 6, 2021 9:46 PM