As an IT support professional, I service multiple business and residential clients. One of my most frequent activities is the installation of operating systems (OSs) on both Mac and Windows computers. As a result, I’ve tended to carry around a number of bootable flash drives, each containing an installer for a different operating system. These include various versions of macOS, Windows, and Linux distributions.

I recently decided it would be much more efficient if I could house all the different OSs I require on a single flash drive. As a result, I created a drive containing a number of versions of the macOS and Windows 10. The drive was created on a Mac running macOS Big Sur but can be booted on either a Mac or a Windows computer. It can then be used to install macOS or Windows 10 on a Mac, or Windows 10 on a PC.

Read on to learn how to create a multi-OS installer flash drive using a Mac.


Preparing the Flash Drive

The first stage in the process is to prepare the flash drive you’re going to use by erasing and formatting it. Following are the steps required.

  1. Select Utilities in Finder and open Disk Utility.
  2. Connect your flash drive and ensure that Show All Devices is ticked in the View menu in Disk Utility.
  3. You should see your flash drive listed under External in the left-hand pane of Disk Utility. Click on it and then click Erase.
  1. Give the drive a Name (I’ve called it OperatingSystems) and select Mac OS Extended (Journaled) as the Format and GUID Partition Map as the Scheme.
  1. Click Erase.
  2. When the erase process completes, click Done.

Writing the First macOS Version to the Flash Drive

You’re now ready to write your first macOS installer to the flash drive. Let’s begin with the most recent version, which is macOS Big Sur.

The first step is to download the installer for macOS Big Sur. Apple has made this pretty straightforward and this page contains links to download all the recent versions of the macOS, together with instructions about how to create a single bootable installer. Just click on macOS Big Sur under the heading Download macOS. This will take you to the App Store where you simply click GET. After a few seconds, you will be asked whether you are sure that you want to download macOS Big Sur and you should click Download.

After the download has been completed you can dismiss any attempt to begin the installation of the macOS. The important thing is that the installer has now been downloaded to your Applications folder. It’s called Install macOS Big Sur.

To write the installer to the desired partition on your flash drive, open Terminal which you will find in the Utilities in Finder. Then type the following command.

sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia –volume /Volumes/OperatingSystems <enter>

This command will be different if you are writing a version other than Big Sur and you should check the exact syntax under the heading Use the “createinstallmedia” command in Terminal on the How to create a bootable installer for macOS page. The other component which may be different will be the name of the volume after /Volumes which in my case is OperatingSystems.

After you press enter you will be asked for the administrator password. You will then need to confirm that you wish to continue, and the operation will proceed. This will take some time, depending on the speed of your flash drive, and you will see the progress from 0 to 100% as the files are copied to the drive.

You now have a bootable flash drive containing the installer for macOS Big Sur. You are ready to proceed with adding additional macOS installers to the drive.

Partitioning the Drive

You first need to partition the drive.

Return to Disk Utility and click on your flash drive in the pane on the left-hand side. You will see that underneath the name of the drive it now has the name of your first volume which is Install macOS Big Sur.

Click Partition and you will be presented with the partitioning window. Within this window, you can define the partitions you require, together with their sizes.

For the moment let’s create one additional partition, which we shall use for the macOS Catalina installer.

Click on the + to create an additional partition and then give it a Name. The Format should be Mac OS Extended (Journaled) as before, and a Size of 12 GB should be adequate.

Click Apply when you’re ready and then confirm by clicking Partition in the next window. The operation will take some time and you should then be presented with a window reporting that the operation was successful. Click Done to dismiss the window.

You can now download the installer for macOS Catalina in the same way as for Big Sur. Then return to Terminal and type in a command similar to that used for Big Sur, but changing the name of the installer and the name of the destination volume. In this case, it would be the following command.

sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia –volume /Volumes/macOSCatalina <enter>

Again, enter your administrator password, confirm that you wish to continue, and the operation will proceed.

You now have a bootable flash drive with two partitions containing installers for different versions of the macOS. You can then continue the process of creating new partitions, downloading the desired installers, and writing them to the flash drive. Apple provides instructions for versions going back as far as OS X El Capitan and it’s unlikely that you would require a version that is older than this one unless you are working with a very old Mac.

Adding a Windows 10 Partition

We shall now add a Windows 10 installer partition to the drive.

Return to Disk Utility, click on your flash drive, and select Partition once again. This time add a partition named Windows10, with format MS-DOS (FAT) and size 8 GB.

Click Apply and then Partition as before.

You now need to download a Windows 10 ISO file (installer) from Microsoft. You can do this by visiting their Download Windows Disc Image (ISO File) page. For the purpose of this guide, I shall assume you’re downloading the 64-bit English International version. Once you have downloaded the file, make a note of its full name in your Downloads folder. Then return to Terminal and type the following command to mount the downloaded ISO file.

hdiutil mount ~/Downloads/<full name of downloaded ISO file> <enter>

Then open Disk Utility and note the name of the disk image you have just created, under Disk Images.

Now, you need to return to Terminal and type the following command.

rsync -avh –progress –exclude=sources/install.wim /Volumes/<name of disk image>/* /Volumes/W10 <enter>

You now need to install a package management tool called Homebrew. To do this, use the following command in Terminal.

/bin/bash -c “$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)” <enter>

You’ll need to enter your administrator password and confirm the operation. Once it has been installed, type the following command.

brew install wimlib <enter>

Finally, enter the following command to write the Windows 10 installer to the Windows partition on the flash drive.

wimlib-imagex split /Volumes/<name of disk image>/sources/install.wim /Volumes/W10/sources/install.swm 3800 <enter>

Testing the Installers

To test that your flash drive functions correctly, first shut down the computer. Then start the computer with the flash drive connected, and hold down the option key as it starts. After a short time, you should see all the bootable options available to you. First will be your Mac hard drive, and then you will see each of the macOS installer versions you have installed to the flash drive, and also the Windows installer, which has the name, EFI Boot. Clicking on any one of these will launch the relevant installer, enabling you to proceed with the installation of that operating system.

Conclusion

As an IT support professional, having a number of macOS installers, together with a Windows 10 installer, on the same flash drive is invaluable. Although we have created the drive using a Mac, the drive can also be used to install Windows 10 on a Windows computer.

Gone are the days of needing to carry around multiple USB flash drives.