How to Create a Bootable macOS Catalina Installer

A bootable version of the macOS Catalina installer is one of the often-overlooked tools a Mac user should have on hand before upgrading to or performing a clean install of Catalina. The bootable installer you create in this guide will be housed on a USB flash drive, but you can use any bootable device you may have lying around, including internal or external drives.

Image of a small OWC flash drive

A bootable flash drive is the preferred method, however; here’s why:

  • It’s small and easy to store in a safe place.
  • It’s inexpensive. You can buy a new USB flash drive large enough for this project for less than $10.
  • All current Macs support booting from a USB port. Macs with Thunderbolt 3 can also use a USB flash drive as bootable media (an adapter may be required).

Why Make a Bootable macOS Catalina Installer?

There are multiple reasons for having a bootable macOS Catalina installer. The four primary reasons are:

  • It allows you to perform a clean install of the operating system on your Mac using the existing startup drive. By booting to the installer, you can perform a full erasure of your normal startup device before installing the new OS.
  • It lets you install the OS on multiple Macs without having to download the installer multiple times.
  • It allows you to archive a copy of the macOS installer, which you may need if you ever want to downgrade to the version on the installer. You may have noticed that with each new version of the macOS that’s released, there are usually some users who have one or more issues with it and wish to return to a previous version. This can usually be done provided you have a copy of the installer for the version of the Mac OS you wish to return to. This bootable installer meets that requirement.
  • It can be used as a bootable troubleshooting tool, letting you access the same apps that are found in the Recovery Volume.

What You Need to Create a Bootable MacOS Installer

The list of items needed to complete this project is fairly basic, and I suspect you may already have everything you need.

A connection to the Internet: You’ll need an Internet connection for downloading the installer from Apple.

A USB flash drive: 8 GB is the minimum size required, at least for all the versions of the macOS through macOS Catalina. Future versions may require larger flash drives, but for now, 8 GB is big enough. Going larger isn’t a bad idea, though; you can use the extra space on the flash drive to store apps and utilities you may need for troubleshooting purposes.

You should also consider using a fast flash drive. A USB 3 flash drive with fast read speeds would be a good choice.

Screenshot of macOS Catalina installer window
Be sure to quit (Command + Q) the macOS Catalina installer if it starts up after the download is completed.

A copy of the macOS Catalina installer.This is usually downloaded via the Software Update preference pane (macOS Mojave and later), or the Mac App Store (macOS High Sierra and earlier). If you’re working with the beta of the OS, you can find the downloadable version either on the Apple developer site, or Apple’s public beta web site.

About 30 minutes of your time: The amount of time it will take is difficult to estimate; making the bootable flash drive is dependent on the speed of the flash drive you’re using, and whether you’re including the time it will take to download the installer, which is dependent on your Internet connection speed and how much traffic exists when you’re downloading. So, take my 30-minute estimate with a grain of salt.

Create the Bootable macOS Catalina Installer

With everything acquired, let’s start by making the bootable installer. Here are a few tips:

Warning: The developer beta and public beta of macOS are not finished products and are actively undergoing tests and revisions to mitigate bugs and improve performance. Beta software can cause unexpected results that could impact the use of your Mac. Before you make use of any beta software you should ensure you have a working backup of your existing system. If possible, beta software should be installed on a test machine and not one used for mission-critical workflows.

When you download the installer, it’s going to automatically launch and offer to start the installation process on your Mac. Be sure to quit the installer; don’t let it start the installation process.

Prepare the flash drive by erasing the drive and ensuring it’s formatted using the macOS Extended (Journaled) file system. You can find instructions in the Rocket Yard guide: How to Use macOS Sierra Disk Utility to Partition, Erase Drives. Although the guide was made for macOS Sierra it should work well enough for erasing a USB flash drive.

We’re going to use the Terminal app to execute an instruction that Apple includes within the macOS installer. The instruction “createinstallmedia” has been part of macOS installers for some time. 

Normally, commands entered into Terminal need to be very precise in order to work. This means the command we want to execute will be different for every version of the macOS for which you wish to make a bootable installer. Since there are likely to be at least three versions of macOS Catalina (a developer beta, a public beta, and a released version), we’re going to use Terminal, along with a drag-and-drop method, to ensure the Terminal command is always correct no matter which version of Catalina you’re going to use.

Terminal and Finder windows for drag and drop creation of Terminal command.
The Terminal app with two Finder windows nearby to simplify
drag-and-drop creation of the bootable installer.

Open a Finder window and navigate to /Applications.

Within the Applications folder, you will find the installer for macOS Catalina. It will have a name similar to “Install macOS Catalina Beta” or “Install macOS Catalina,” depending on whether you’re using the beta or the release version.

Once you locate the installer, right-click on it and select Show Package Contents from the popup menu.

The Finder window will change to show the internal contents of the installer. Open the Contents folder and then the Resources folder. Scroll through the list of items in the Resources folder until you come to the createinstall media.

For now, set this Finder window aside, putting it someplace where it is visible on your desktop.

Open a second Finder window by selecting File, New Finder Window from the Finder menu.

From the Finder menu, select Go, Go to Folder.

In the sheet that opens enter /Volumes and then click the Go button.

You should now have two Finder windows open, one showing the content of the Resources folder on the macOS installer, and the newest one, showing the volumes that are attached to your Mac.

In the list of volumes locate the USB flash drive you prepared earlier. Its name may be Untitled, or it could have a name you gave it when it was erased. I chose to name mine Flash Drive, making it easy to find. If you’re not sure, you can always return to Disk Utility, erase the USB flash drive again and give it a distinctive name.

With the two Finder windows ready, it’s time to open the Terminal app:

Launch Terminal, located at /Applications/Utilities.

Terminal will open and display a prompt waiting for you to enter a command. Instead of typing a long command string you will build the command using drag and drop.

Warning: The createinstallmedia command will erase all content on the flash drive.

Type the following at the Terminal prompt:

sudo

Add a space after the word sudo, Do not press the Enter or return keys.

Image showing dragging createinstallmedia from the Finder window to Terminal window.
Drag the createinstallmedia item from the Finder to the Terminal Window.

From the Finder window showing the content of the Resources folder, drag the createinstallmedia file to the Terminal window and drop it on the prompt. This will create a long entry that begins with /Applications/Install\ macOS\ Catalina\ and ends with /createinstallmedia. 

Type the following in Terminal after createinstallmedia;

–volume

Make sure you enter a space after the word volume and do not press the Enter or return keys.

From the Finder window showing the content of the Volumes folder, drag the USB flash drive to the Terminal window.

Type the following in Terminal after the USB flash drive name:

–downloadassets

The completed Terminal command should look something like this if you’re using the Catalina beta: 

sudo /Applications/Install\ macOS\ Catalina\ Beta.app/Contents/Resources/createinstallmedia –volume /Volumes/Install\ macOS\ Catalina\ Beta –downloadassets

Or, if you’re using the release version of Catalina, it will probably look like this:

sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia –volume /Volumes/Install\ macOS\ Catalina –downloadassets

Note:The –volume and —downloadassets entries should be entered as two dashes and then the parameter name. Sometimes our blog software will convert a double dash to a single long em dash.

The completed Terminal command to create a bootable installer for macOS Catalina.
The complete Terminal command for creating a bootable installer of the macOS Catalina beta.

You can now hit the Enter or return key.

Terminal will ask for your password. This is your administrator account password for your Mac. Enter the password and hit the Enter or return key.

You’ll be asked if you wish to continue and erase the flash drive; type Y at the Terminal prompt, then hit return or enter.

Your flash drive will be un-mounted and its contents erased; the process of creating the bootable installer will then begin.

Once complete, Terminal will display:

Install media now available at “/Volumes/Install macOS Catalina”

Terminal executing the createinstallmedia command.
Terminal will keep you informed about the progress of creating the bootable macOS Catalina installer.

Your flash drive is now ready to be used as a bootable installer for macOS Catalina.

Alternatives to Using Terminal

If you’re not comfortable using Terminal to create the bootable installer there’s another option. DiskMaker X is an app that can build the bootable installer for you. The current version DiskMaker X 8.0.3 is designed for use with macOS Mojave. The DiskMaker team usually updates the app after the release version of a new Mac OS is released.

If you need to make a bootable installer of the beta versions of the macOS, Terminal is your best choice. If you want a bootable installer of the released version of macOS Catalina, you can use either Terminal, as described above, or DiskMaker X, once it’s updated.

Using the Bootable Catalina Installer

The bootable installer is used like any other bootable device connected to your Mac. Restart your Mac and immediately hold down the Option key as your Mac boots.

After a few moments the Mac’s boot manager will display all the bootable devices connected to your Mac. Use the arrow keys to select the USB flash drive (likely named Install macOS Catalina).

The boot process will continue, eventually displaying the macOS Utilities window.

macOS Utilities window.
macOS Utilities are displayed when you start up from the bootable macOS Catalina installer.

At this point, you can choose to:

  • Restore From Time Machine Backup: If you have a system backup, you can choose this option to restore from Time Machine.
  • Install macOS: You can choose to install macOS Catalina.
  • Get Help Online: Browse the Apple support site for help
  • Disk Utility: Use Disk Utility to erase, repair, format, or partition storage devices.

You can also use the Utilities menu to gain access to:

  • Startup Security Utility
  • Network Utility
  • Terminal

Using the various macOS Utilities is beyond the scope of this guide, but a common option is to use Disk Utility to completely erase and format the Mac’s internal startup drive, and then use Install macOS to perform a clean install of macOS Catalina.

Warning:A clean install will erase everything on the selected startup device.



LEAVE A COMMENT


  • There used to be a way just to find the installer before it completed the install in the utilities folder, which you could drag to a flash drive. Is that still an option?




  • I am a little dense sometimes but I don’t understand how you get the installer for MacOS Catalina file into your Applications folder. It certainly doesn’t exist in mine now. You go right from talking about the Terminal Application to finding the Catalina installer file in my Applications folder. Did I miss something?




    • You can download the installer from the Apple website, either from the Mac App store, the Software Update service, or from the developer or beta users website.

      Once the download completes, the installer will be found in the/Applications folder.

      Tom




  • Apple is blocking any chance to update even from Sierra to HighSierra when you change your HD or SSD




  • I’ve run into trouble with bootable USB macOS installers with either;

    a certificate failure

    a message saying that I could’nt install an older version of the OS

    Does your method factor in these issues?




  • Or just download the installer and install on any disk, including external portable SSD or pendrive.




  • If you need to make more than one installer drive, a short shell script is a lot easier.
    ——–
    #!/bin/bash
    # make macos bootable installer thumb drive on $Volume, using macos
    # install app at $AppHome (default High Sierra)
    # takes about 6 minutes on an unloaded 4-core iMac

    AppHome=’/Applications/Install macOS High Sierra.app’ # default High Sierra
    Volume=’/Volumes/NO NAME’ # default SanDisk name

    echo “To accept a default in parenthesis, enter RETURN at prompt.”
    echo -n “Enter pathname of install image ($AppHome):”
    read line
    if [ “$line” != “” ]; then AppHome=”$line”; fi
    echo -n “Enter pathname of volume ($Volume):”
    read line
    if [ “$line” != “” ]; then Volume=”$line”; fi

    sudo “$AppHome”/Contents/Resources/createinstallmedia –volume “$Volume” –applicationpath “$AppHome” –nointeraction




    • Thanks, Ed
      A pretty nice shell script. For those who wish to use Ed’s script be aware that our blog software has changed the double dash this is needed in front of the volume, applicationpath, and nointeraction options into single em dashes.

      Also, the applicationpath option is not required for macOS Catalina version of the createinstallmedai command allowing you to change the last line of the script to:

      sudo “$AppHome”/Contents/Resources/createinstallmedia –-volume “$Volume” –-nointeraction

      Tom




  • Does the T2 chip require changing to allow external boot device?




    • For those Macs with a T2 security chip, you need to enable the option to allow external booting. This can be done from the Startup Security Utility:

      Boot your Mac while holding down the Command + R key to enter Recovery mode.

      Select the Startup Security Utility from the Utilities menu.

      Select the option to Allow booting from external media.

      You should then be able to boot from the USB flash drive you created to install macOS Catalina.

      Tom