Tech Tip: How to Use Boot Camp on an External Drive

Boot Camp IconBoot Camp and Boot Camp Assistant allow you to install Windows on your Mac. It’s a nice capability that lets you select – at boot time – which operating system you wish to use: macOS or Windows.

One of the downsides to Boot Camp and the Windows installer is that it restricts you to installing Windows on your Mac’s internal drive. While Boot Camp Assistant can partition your startup drive for you to make room for Windows, there are bound to be many of you who just don’t have room to spare on your startup drive to install Windows.

Installing Windows on an external drive would be a great solution to the problem of available space, but as we said, Boot Camp and Windows impose a restriction on installing to an external drive – or do they?

There are actually a few ways you can successfully install Windows on an external drive. They range from creating clones of an existing PC installation or using Microsoft IT tools for installing Windows. But the method we’re going to outline here is a bit different. It allows you to install Windows on an external drive without first having Windows installed on a PC or in a virtual environment.

This is an advanced process with quite a few pitfalls that can trip you up. Be sure to read through the process before undertaking it. Also, make sure you have a current backup before beginning.

What You Need

  • 4 GB or larger USB flash drive.
  • Bootable external drive. We’re using a USB 3.0 drive, but a Thunderbolt drive should work as well.
  • Boot Camp Windows Support Software.
  • VirtualBox virtualization app (available for free).
  • A licensed copy of Windows 10 ISO or an install DVD.
  • Wired keyboard and mouse. During the Windows installation, the drivers for Apple wireless keyboards and mice aren’t installed until the very end of the process. If you’re not using a portable Mac with a built-in keyboard and trackpad, you’ll need a wired keyboard and mouse to complete the installation.

Prepare the External Drive for Installing Windows
The external drive that you’ll install Windows on needs to be prepared by erasing and formatting the drive for use with Windows.

Warning: The erase, format, and partitioning process will delete all data currently contained on the external drive.

1) Ensure the external drive is connected to your Mac.
2) Launch Disk Utility, located at /Applications/Utilities.
3) In the Disk Utility sidebar, select the external drive that will be used for Windows. The device will likely have the name of the external enclosure’s manufacturer listed. Do not select the volume located just underneath the device name.
4) Click the Erase button in the Disk Utility toolbar.
5) Use the dropdown menu to set the Scheme to “Master Boot Record.”
6) Use the dropdown menu to set the Format to “MS-DOS (FAT).”
7) You can use any name you wish (up to 8 characters), but we suggest naming the external WIN10.
8) Click the Erase button.

The external drive will be formatted and a single MS-DOS (FAT) volume will be created.

Prepare the USB Flash Drive for the Windows Support Software
The USB flash drive needs to be formatted to accept the Windows software that Boot Camp Assistant will download and install.

Screenshot of flash drive formatting window

(The flash drive needs to be formatted for use on Windows.)

Warning: The process of preparing the USB flash drive will delete any data contained on the flash drive.

1) Make sure the USB flash drive is connected to your Mac.
2) Launch Disk Utility, located at /Applications/Utilities.
3) In the Disk Utility sidebar, select the USB flash drive device. The device will likely have the name of the USB flash drive manufacturer listed. Do not select the volume located just underneath the device name.
4) Click the Erase button in the Disk Utility toolbar.
5) Use the dropdown menu to set the Scheme to “Master Boot Record.”
6) Use the dropdown menu to set the Format to “MS-DOS (FAT).”
7) You can leave the Name field as is.
8) Click the Erase button.

The USB flash drive will be formatted and a single MS-DOS (FAT) volume will be created.

Download Boot Camp Windows Drivers
We only need Boot Camp Assistant for its ability to download all of the Windows drivers we’ll need to ensure the install of Windows will run correctly on Mac hardware. We won’t be using Boot Camp Assistant to partition a drive or step us through the install process.

Screenshot of boot camp assistant

(The latest version of the Apple drivers that Boot Camp needs can be downloaded using Boot Camp Assistant.)

1) Launch Boot Camp Assistant, located at /Applications/Utilities.
2) In the Boot Camp Assistant window that opens, click the Continue button.
3) In the Select Tasks window, remove checkmarks from “Create a Windows 7 or later version install disk” and “Install Windows 7 or later version.” Make sure the checkbox labeled “Download the latest Windows support software from Apple” is checked, and then click Continue.
4) Select the device you wish to have the Windows support files installed on; this should be the USB flash drive you prepared earlier. After making your selection, click Continue.
5) Boot Camp Assistant will download and install the needed Windows support software on the selected device.
6) Once the install is nearly complete, you’ll be asked to provide your administrator password so the Boot Camp Assistant can change the file permissions on the USB flash drive. Provide your administrator password, and click Continue.

The Windows support software has been installed on the USB flash drive.

Use VirtualBox to Install Windows on the External Drive
This is the tricky part of the process, at least in the sense that we’re going to trick Boot Camp and the Windows installer into thinking your external drive is actually your main internal drive, or in the parlance of Windows, your C: drive.

You could perform this tricky bit of virtualization using Parallels or VMware Fusion, but we’re going to use VirtualBox because it’s free. You can download the app from the VirtualBox website.

Once you download and install VirtualBox, we’re ready to begin the installation process.

Most of the virtualization trickery will be performed from within Terminal, so go ahead and launch Terminal, located at /Applications/Utilities.

Screenshot of diskutl command

(You can find the external drive by looking for the name, type, or if this is your only external, by the location [external, physical]).

1) With the external drive that you formatted for installing Windows on connected to your Mac, enter the following Terminal command without the quotation marks: “diskutil list” and then press enter or return.
2) A list of all attached disks will be displayed in Terminal. Scroll through the list and locate the external drive you plan on using to install Windows. If you followed our suggestion earlier, it will be named WIN10, and will be of the type DOS_FAT_32.
3) Once you locate the external drive, make a note of its Identifier. The Identifier appears in the last column and will have the format of the word “disk” followed by a number. In our case, the identifier is disk4.
4) Now that we know the disk identifier, we need to eject the disk so it’s no longer connected logically to the Mac (it will still be connected physically).
5) Locate the WIN10 disk on your Desktop or in the Finder window sidebar.
6) Right-click on the WIN10 disk and select Eject from the popup menu.

Use VirtualBox to Map the External Drive to a VirtualBox Disk
The next step in the process is to map the external drive to a VirtualBox disk.

1) In Terminal, enter the following without the quotation marks: “sudo VBoxManage internalcommands createrawvmdk -filename “bootcamp.vmdk” -rawdisk /dev/disk4″ Note: Be sure to replace /dev/disk4 with the actual disk number you found from the disk identifier, and then press enter or return.
2) At the Terminal prompt, enter your administrator password, and then press enter or return.

The bootcamp.vmdk virtual disk will be created in your home folder.

Create a VirtualBox Virtual Machine for Windows
We will use VirtualBox to install Windows 10 to our external drive. We can do this because in the previous step, we mapped the external drive to a VirtualBox virtualized disk.

Virtual Box VM

(The Windows VM you create uses a virtualized connection to your external drive.)

In order for VirtualBox to be able to access the external drive, we need to launch VirtualBox with elevated permissions. Once again, we turn to Terminal.

1) Enter the following at the Terminal prompt without the quotation marks: “sudo /Applications/” Press enter or return.
2) If asked, supply your administrator password, and then press enter or return.
3) VirtualBox will open.
4) In the VirtualBox window, click on the New button in the toolbar.
5) In the sheet that drops down, enter a Name for the installation; we suggest WIN10.
6) Set the Type to Microsoft Windows.
7) Set the Version to Windows 10 (64-bit).
8) Click the Continue button.
9) Accept the default memory size, and click Continue.
10) In the Hard Disk sheet, select Use an existing virtual hard disk file.
11) Just below the option to use an existing virtual hard disk file is a dropdown menu for selecting a file to use. You may notice the menu is empty or does not contain the virtual disk file we created earlier. Click the folder icon just to the right of the dropdown menu. This will allow you to browse to the bootcamp.vmdk file, which is located in your home folder. Select the bootcamp.vmdk file, click the Open button and then click the Create button.
12) VirtualBox has created a virtual environment for us to install Windows 10 in that will actually perform the install on the external drive. The next step is to mount the Windows 10 ISO you downloaded earlier from Microsoft.
13) In VirtualBox, select the Windows 10 virtual machine, and then locate the Optical Drive. This is usually located in the Storage section of the virtual machine. Click the Optical Drive item and select Choose Disk Image.
14) Browse to where you downloaded the Windows 10 ISO.
15) Select the Windows 10 ISO, then click open.

Install Windows on Your External Drive Via VirtualBox
1) Start the installation process by clicking the Start button in the VirtualBox toolbox.
2) The Windows 10 installation will start. Follow the onscreen instructions until you come to the Windows Setup screen with the heading “Which type of installation do you want?”
3) Select the “Custom: Install Windows only (advanced)” option.
4) The Windows setup will display the currently available drives and partitions that Windows can be installed on. Because the external hard drive has been assigned to the VirtualBox machine, it is the only drive that will be listed.
5) When we used Disk Utility to format the drive, we chose MS-DOS (FAT), which is incompatible with Windows 10. We did this because Disk Utility can’t format with NTSF, but the Windows installer would recognize MS-DOS. All we need to do now is change the drive’s format to NTSF.
6) Select the drive, and then click the Format button.
7) Wait until the format is complete (the Next button will become available), and then click Next.
8) The Windows installation will start, with files being copied to the external drive.

Warning: Extremely important step follows.

9) When you see the message “Windows needs to restart” immediately shut down the virtual machine by clicking on the red close button on the window. Select “Power off the machine” from the list of options, and then click OK to power off the virtual machine.

Windows 10 Install

(Windows Setup will install the needed files to your external drive. Be sure to prevent Windows Setup from automatically restarting.)

At this point, the Windows installer has copied all the files to the external drive, and has set up a boot environment that you can start your Mac from. Next time you boot from the external drive, Windows will complete the installation process.

Restart Your Mac With the External Windows Drive
1) Close any apps you may have open, then restart your Mac.
2) Hold down the Option key during the restart. This will cause the Mac’s Startup Manager to display a list of drives you can start from. Use the arrow keys on your keyboard to select the Windows drive.
3) Your Mac will boot from the external Windows drive. The bootup process can take a while, so be patient.
4) Windows will finish the install process. At some point during the installation, Windows will restart your Mac. When it does, your Mac will restart with the normal Mac OS. You need to be present to hold down the Option key and select Windows to start from.
5) Windows will finish the installation and present you with the Windows desktop.

Getting Apple Hardware Working Under Windows
The USB flash drive you used earlier to make the Windows Support software contains all the drivers you need to install in Windows for the Apple hardware to work.

Boot Camp Setup screen

(To complete the Windows installation, run the Boot Camp Setup app to install the needed Apple drivers.)

1) Make sure the USB flash drive is connected to your Mac.
2) Click on the Windows Start button and select File Explorer.
3) In the File Explorer sidebar, select the USB flash drive, it will probably have a D or E drive letter assigned to it.
4) Open the Boot Camp folder
5) Run the Setup app inside the Boot Camp folder.
6) Follow the onscreen instructions to install the Boot Camp drivers.

Once the installation completes, all your Apple hardware, including wireless keyboards and mice, should be working.


  • Did someone get it to work on a 2019 MacBook Air? It installs correctly and USB mouse works but not all the hardware. WLAN did work with BootCamp installed and I got some drivers but no trackpad or internal keyboard support.

  • what happens if you unplug the usb hard drive and you are on windows?

  • Installation succes but after i shutdown and on the macbook and hold the option button theres no other drive. Only macintosh hd. Please help me to fix. Thank you.

  • Hello, im trying to install win10 on mac Mojave 10.14.6

    I followed the instructions and the issues in the comments until the boot device showed up

    but after running it. Mac log will show and and box telling me that i need to reinstall the mac OS.

    one thing that didnt work for me and I dont know if it will effect the process. while installing win10 on the hard disk I cound not Format the disk from inside win.

    EFI and O/I are enabled. the first erase for the Hard drive is GUID MS-DOS (FAT)

    Thank you

  • I wonder if this would work with a partition in the internal drive. I have a SSD drive which is not the original Apple one (it was too small) but the new one does not support Bootcamp. I wonder if this procedure may help me install Windows 10 in a different partition of internal disk without using bootcamp

  • I managed to follow the process but in the end, when I powered off the virtual machine and restarted holding Options key, my mac was trying to install win 10, everything seemed good until bluescreen appeared…and restarted in a flash that I even couldn’t read the error message…
    Any suggestion please?

  • Can someone tell me the purpose of installing and running Windows from an external drive instead of internal drive?

    The reason I ask, I have a Macbook Pro from 2009 that is too slow/underpowered to run Windows 10 on VirtualBox. If I use an external drive for Windows instead, will Windows run quicker when connected to the Macbook?

    • You might not have a ton of space on your internal drive, or for me I have 2 computers and I only use Windows for specific software, so it makes sense to be able to carry it with me. If you have an external SSD, and your internal drive is a hard disk, I think it should be faster. But USB is not as fast as SATA, so if they’re both the same (SSD or HDD), maybe not.

  • The first virtualbox command failed with VERR_FILE_NOT_FOUND. I tried with the thumb drive still connected, and got a busy error. Solution was to use disk utility and eject just the partition (disk2s1), leaving the drive (disk2) alone.

  • Failed to open a session for the virtual machine WIN10.

    The virtual machine ‘WIN10’ has terminated unexpectedly during startup with exit code 1 (0x1).

    Result Code: NS_ERROR_FAILURE (0x80004005)
    Component: MachineWrap
    Interface: IMachine {5047460a-265d-4538-b23e-ddba5fb84976}

    Please HELP

  • So the whole process worked great for me. No issues.

    But is it possible to start up Virtual Box with this Windows computer I now have contained on a USB drive?

  • Hi all,

    I followed these steps and the one’s included on Sven’s Medium page. However, once I restart the Mac then input my Wireless info and select the ‘Boot EFI’ drive to boot into it immediately goes to the Apple logo, loads, then displays a message in Boot Recovery Assistant that ‘An Internet connection is required to verify this start up disk’. Even after reconnecting to my network it just boots into OS X.

    Has anyone else experienced or solved this issue?

    2018 Macbook Pro
    Mojave 10.14.3

    Samsung T5 SSD

    VirtualBox 6.0.10

    Windows Home 64-bit, newest ISO release


  • This worked finally. I have to try multiple times with different hits and tries. For my case, I am writing down what i did, if it helps anyone here.

    Laptop model: Macbook pro mid 2012 retina
    External hard disk used: 160 GB Toshiba (Actually taken out from my PS3 console :p)

    Failures and their alternatives followed:

    Downloading Boot Camp Windows Drivers on external usb always failed for me. For some reason, after downloading for 30 minutes and at an end, the Boot Camp Assistant gave error unable to save windows support software on selected drive.
    When starting Boot Camp Assistant, “Download Windows Support software” option is in Action menu on top and it asks for saving it anywhere on your mac drive. Downloaded it locally and then manually copied it to my USB external drive.

    When Creating bootcamp.vmdk, I was getting I/O read error showing as its already in use.
    Make sure the External Drive on whom you want to install windows is ejected and run the command in terminal again.

    After bootcamp.vmdk creation, my mac automatically mounts the external drive. which caused it not to be used in virtual box installation.
    After running command on terminal for rawdisk bootcamp.vmdk creation, again eject your drive and don’t include quotes around bootcamp.vmdk

    I was using some old Win10 ISO lying around and it was not booting on mac after all steps and installation.

    Download official win10 64bit iso from windows site. It worked for me.

    After completing all steps, installed bootcamp drivers on windows, rebooted couple of times, everything worked except I am unable to open Bootcamp control panel on windows 10 to configure touchpad click and sensitivity etc.
    It gave error “Bootcamp can’t access startup disk..”

    I was not able to find its solution so far. Maybe its because bootcamp expects the installation of windows to be on MAC drive rather then on external drive.
    Any help in solving this problem will greatly help.

  • This worked for me after trial and error with some of the updates on the procedure. I installed in a Samsung T5 1TB SSD, using a MacBook Pro 13″ 2017 version.

    after successfully making everything work I went greedy. I decided to shrink the 1TB partition within Windows, so I could re-purpose it and reuse it in MacOS. I created a G: approx. 500MB large as simple volume of exFAT. I then went into MacOS, where G was visible and I instructed disk utility to erase that and make it MacOS journaled etc. I got an error about not enough space, and after that, when I boot the MacBook I don’t get the window10 boot drive when I hold down option key. I’m suspecting perhaps disk utility deleted the EFI boot partition Windows had created? I’m lost. Now Windows 10 is locked inside the SSD and I am not sure how to access it from my Mac…

  • How can you do this with Parallels

  • Failed to open a session for the virtual machine WIN10.

    VD: error VERR_NOT_SUPPORTED opening image file ‘/Users/fatehsyed/bootcamp.vmdk’ (VERR_NOT_SUPPORTED).

    VD: error VERR_ACCESS_DENIED opening image file ‘/Users/fatehsyed/bootcamp.vmdk’ (VERR_ACCESS_DENIED).

    Failed to open image ‘/Users/fatehsyed/bootcamp.vmdk’ in read-write mode (VERR_ACCESS_DENIED).

    AHCI: Failed to attach drive to Port0 (VERR_ACCESS_DENIED).

    Result Code: NS_ERROR_FAILURE (0x80004005)
    Component: ConsoleWrap
    Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}


  • When i restart my mac, the external drive is not shown as an option to boot from – how can i fix this?

  • I got to the reboot point, and the WIN10 drive won’t show up, seems like others also have this issue, but I don’t see a solution. Anyone know how to get the EFT partition in there? I don’t know the deal with that.

    • There are many, many posts below in comments for that exact issue, all the way back to the earliest comment. I even left help about it a couple or few times. Good Luck!

  • Hello when I try to select the bootcamp.vmdk It says failed to open the disk image file

  • Hi, thanks for the tutorial! If I am using Parallels instead of VirtualBox how is the
    “sudo VBoxManage internalcommands createrawvmdk -filename “bootcamp.vmdk” -rawdisk /dev/disk4”
    command changed? Does it change other parts of the process? Any help is appreciated!

  • Great jobs. I followed the procedure and now windows 10 pro works perfectly in my iMac 21,5 late 2013. I’ve installed windows in a Samsung SSD t5 250 GB.
    Thank you very much.

  • In VirtualBox I cannot select the bootcamp.vmdk file — it is greyed out

  • This procedure does not work. I have tried it multiple times and follows the procedures exactly.

    A 2019 iMac 4k (6 core/16GB/256GB) will NOT boot off of a USB drive. I have a Samsung SSD T5.

    • I think would be helpful to know I’d this is because it’s a 2019 and that’s how Apple designed their new Macs or not, because I have successfully done this procedure twice in the past year, on a 2015 MacBook Air Mojave, and a 2012. T5 is good because it installs the smoothest to that SSD. However, I did not follow the procedure “exactly”. I don’t believe the original writer updates the instructions anymore, and early on relied on replying to comments and questions as his update.

      Here is what I do and what I suggest anyone do: I first read through all the comments below, all of them, right back to the time it was originally posted because the writer answers in many of those and added additional instructions or explanations. I printed off the original instructions and made notes on each section where comments by others mattered or changed it. Then I attempted to do it. I ran into some things as most people will with so many different models and peripheral specs, but they were easily resolved by either the comments below, or a bit of trial and error. Three days is the longest the first install took, the second one was an hour and a half.

      It does work. But I think you need to pay close attention to many comments and questions below and adjust the instructions accordingly. It is well worth that effort and extra time.

  • I have come to the last part of the tutorial, but everything goes perfect, but I can’t choose the boot drive when I’m booting the computer again??? Please help. I can’t figure it out.

  • I’m running into an issue for the final step. I got everything completed, Windows is running on my external drive, but installing the drivers is where my issue lies.

    When I run the setup app from my USB the setup seems to be going fine, and then the screen goes black. When I move my cursor to wake up the screen its really dim. If I let the setup finish it messes up my Windows install, so when I try to start up Windows again I just get the error status that it can’t boot, or something like that. When I look on my external drive the Windows files seem to still be there, but Windows won’t work.

    Does anyone have any ideas why installing the Bootcamp setup would do this?

  • I’ve done everything to the letter but still not working. My setup is below:
    Macbook Pro 2018
    OSX 10.14.4
    External Drive – OWC Express
    Drive in enclosure – Intel 320 Series SSD 300GB
    Cable for external drive – AmazonBasics USB Type-C to Micro-B 3.1 Gen2 Cable – 3 Feet (0.9 Meters)
    Here’s what I’m experiencing:

    1. Post VirtualBox installation and restart, I can’t only see the OSX Drive, EFI Boot but no windows
    2. Tinkering around in the Startup Utility gives me the Bless Tool issue (it is not a blessing)

    Any ideas for a fix?

    • It probably was the EFT boot drive. Mine at first didn’t say Windows. It said EFT Boot. Have you tried selecting that at boot to see what happens?

    • I’m having the same problem.
      MacBook Pro (15-inch, 2016)
      macOS 10.14.4
      External Drive – Delock External Enclosure For M.2 NVMe PCIe SSD – USB 3.1 Gen2 Type C
      Drive in enclosure – Intel SSD 660P 1.0TB M.2 80mm PCIe 3.0

      Here’s what I’m experiencing:
      1. Post VirtualBox installation and restart, I can’t only see the OSX Drive, EFI Boot but no windows

      I haven’t tried to select it in Startup Utility and boot.

      I’ll maybe try to do the installation on a normal PC and try the post installation on the Macbook.