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: Mac OS 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.

(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.

(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.

(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.

(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 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 boot up 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.

(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.


  • What Mac operating system version is he using?

  • Thanks for the tutorial. Everything was smooth until the reboot, after it I always get the same message “INACCESIBLE BOOT DEVICE”. Has anybody came acrossthis issue? Any clues? Thanks

  • Nice tutorial, just some observations:

    1. The correct command to create the virtual disk in the external drive is not:

    “sudo VBoxManage internalcommands createrawvmdk -filename “bootcamp.vmdk” -rawdisk /dev/disk4″

    The correct command is:

    “sudo VBoxManage internalcommands createrawvmdk -filename bootcamp.vmdk -rawdisk /dev/disk4″

    2. If VBox is not allowing you to choose the drive in your home folder, eject the external drive from macOS and then it should let you.

    3. This method only works with computers that have legacy BIOS enabled. Most recent computers that have EFI boot mode(like Mac from late 2015 or newer) will not be able to see the hard disk at startup and will not allow you to boot from it because when Windows was installed, there was not EFI partition created. Either find a way to emulate EFI mode in VBox so the installer can automate this process or install Windows to the drive in an EFI enabled computer instead of VBox.

    4. To download Windows support software, all you need to do is run boot camp assistant and click on the Actions menu. you’ll see and option to download the support software there.

  • O followed all the steps, but when I try to restart to Windows, it show the message: No bootable Drive
    What did I do wrong? Did anyone have this problem too?

  • even though I entered virtualbox through the sudo command as directed, when trying to create the virtualbox it was unable to access the bootcamp.vmdk file giving me a VERR_ACCESS_DENIED error as follows “Failed to open a session for the virtual machine WIN 10 2.

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

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

    Failed to open image ‘/Users/milesleong/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}”

    Anyone know why this is happening and/ or how to fix it? I feel like this is a somewhat simple permissions issue that I just can’t seem to figure out. Thanks!

  • When I open boot camp assistant it just tells me to partion my macs hard drive for windows it doesnt come up to select tasks

  • so this was working fine for me until I got to this step:
    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.

    I tried locating it, but then VBox crashed and gave me an error in the terminal:
    2017-09-06 15:36:17.940 VirtualBox[11201:3227065] Couldn’t find or read strings file SlicesStrings
    Assertion failed: (isSoftlinked), function ___ZN2NL8Resource19createAssetResourceEPK10__CFLocalePK10__CFStringS6__block_invoke, file /Library/Caches/, line 48.
    Abort trap: 6

    so I tried again, but got a different error when I repeated step 8:
    “Cannot create the machine folder WIN10 in the parent folder /var/root/VirtualBox VMs.

    This folder already exists and possibly belongs to another machine.”

    I tried locating this to delete it and continue, but I can’t access /var/root to do so. I’m very frustrated because I’m in a bit of a hurry to get this whole thing set up. please help. :(

    • I had the same problem. I ejected my hard drive and that seemed to fix it for some reason. I had another weird similar problem with virtual box later and did the same thing and it seemed to work.

  • I am having the issue of not being able to see the SSD when restarting my Macbook Pro running Sierra 10.12.6. I am using a WD My Passport 512 and am not sure if it is the SSD that just isn’t compatible or what not. I am not able to see the SSD as an optional bootup disk when I restart the computer holding the option key.

    I have been searching through the comments and have not found a solution that works for me. I don’t have a PC so I am only able to use the VirtualBox to format and prepare the SSD.

    Any help or recommendations would be nice.

  • For me, the terminal says “invalid parameter”… I removed the quotes and have the right disk number. Any suggestions?

  • Windows installed and boots when Option pressed. I’m not seeing the Boot Camp drive (WIN10 E) to load apple drivers. Any ideas why this isn’t showing up to run setup to install drivers? If I try to run Bootcamp Assistant again, it complains that I need a Fat partition.

  • Everything worked well, only the external drive is not showing up when restarting my Mac.

    I have a 2016 MacBook Pro on Sierra. The external drive is a sandisk extreme 900 SSD. Formated as MBR.

    I also tried with the GUID partition format and EFI option enabled in virtual box, but then windows will not allow the installation on the disk.

    Apparently different people have the same problem, but I don’t see a solution in the feedback.

    Any ideas?

  • Just want to comment, when it says the bootcamp.vdmk is in my ‘home’ folder, where is my home folder? (not a tech genius)

  • Thanks, worked great!

    for anyone having trouble with making the bootcamp.vmdk, be sure to remove the quotes around bootcamp.vmdk or you’ll get the VERR_FILE_NOT_FOUND error

  • I’m having the issue where the Windows OS drive is not recognized as a startup drive at the step just after the drive is prepared with the Windows 10 install in VirtualBox. I saw a mention about EFI but not sure how that would work. I also saw the video about this process but they require a PC, which I don’t have. I also tried setting the startup disk as “windows untitled” in the Startup Disk system manager without any lucky. Does anyone know of any other solutions I might try? Thanks.

  • Hi, i managed to get the windows disk showed when booting while holding the option key, but as soon as i start to boot to windows, it shows error missing winload.efi. any thoughts? thanks

  • I ran the following:

    sudo VBoxManage internalcommands createrawvmdk -filename “bootcamp.vmdk” -rawdisk /dev/disk1

    After entering my password get the below error:

    VBoxManage: error: Cannot open the raw disk ‘/dev/disk1’: VERR_FILE_NOT_FOUND
    VBoxManage: error: The raw disk vmdk file was not created

    What do I do?

  • So, I did it all, but – MacPro late 2013 cannot start – windows drive just doesnt show, when i press option on startup. the most interesting part is, that on MacBook Pro and on Windows machine it DOES show, and starts.

  • running into a problem @Create a VirtualBox Virtual Machine for Windows, step 14&15, the ISO I downloaded isn’t showing up at all.

    • OK so I found the ISO. I put it on my desktop thinking it would be easy to find that way but apparently there are several “desktop folders” and I had the wrong one. Stupid mistake.

      However, I have no completed the installation process but when I try to boot holding the option key, the only choice of boot drives I get is MacHD, and the windows drive isn’t visible. I’ve gone through this entire procedure several times trying several different variations(such as using a GUID instead of MBR, enabling EFI), but nothing makes my windows drive choosable at startup.

      when I use system prefs to choose the start disk is says: “the bless tool was unable to set the current boot disk”.

      any ideas as to what’s going wrong here?

  • I’ve followed every step of the way, but I could not boot up Windows after restarting my mac.

    My OS is sierra 10.12.3.

    What should I do?

    • I had this same problem just now. I stopped the virtual machine like the “warning: extremely important step” mentions. I closed all mac OS apps and restarted the mac while holding the option key. I connected to wifi prior to choosing which operating system to run, then started windows OS. It finished installation after a while, then restarted. When i held option this time, i could not see the windows OS partition so I booted to mac OS. Once in mac OS, I clicked on the apple logo in top left corner, clicked on system preferences, then clicked on startup disk, then clicked on a folder icon titled “untitled windows” – this icon does not look like the mac OS hard drive icon. The windows OS icon should be highlighted after you click it (may have to click the lock button on bottom left to unlock the settings). click “restart” and the computer should shut down and start in windows, then you can continue to follow initial setup on windows. I’ll update this post if i find any other hints/tips.

  • When reboot with alt key, there is no bootcamp available, only Mac HD

  • Hi, I’m receiving an error when I’m entering the vbox command in terminal:

    sudo VBoxManage internalcommands createrawvmdk -filename bootcamp.vmdk -rawdisk /dev/disk2

    Cannot open the raw disk ‘/dev/disk2’: VERR_FILE_NOT_FOUND

    Am I missing something ?

  • Everything’s been working great…..

    except for when I try to restart my Mac from the external drive, holding option after the restart…. I don’t see the drive there. Only the MacHD.

    Any thoughts?

    • has to do with it not being EFI enabled at startup. you can click this when creating the install under Virtualbox but then it just goes to a shell during install. Stuck now, uncheck EFI and it installs but doesn’t recognize during option boot or check it and you can’t complete an install. catch 22

    • You could try going to system preferences (on your normal drive) -> startup disk then checking for it there. If it’s there try restarting it from system preferences

  • If anyone is having an issue with selecting the .vmdk file from ‘home’, change the “sudo VBoxManage internalcommands createrawvmdk -filename “bootcamp.vmdk” -rawdisk /dev/disk4”. Remove the “” from bookcamp.vmdk and remember to change the disk number.

    If it still doesn’t work, make sure that your external hard drive is not connected i.e. right click on it and eject it.

  • Just a note to the author. I noticed in the step where we identify the disk# in the Terminal that the instructions refer to disk4, however the screenshot indicates disk3. I found that part a bit confusing! The rest is crystal clear.
    Great work and thanks for the write-up.

  • I installed Windows10 pretty much using the same process (had VMWare Fusion already so used that) sometime back and all was fine and dandy.

    However, trying to update 10 Creators Edition simply is not working. Whether using the Update Tool or the ISO image, either directly in the Win10 installation or from using as a virtual machine, it will always error out basically saying you can’t install update to USB drive (BTW, my install is on a Thunderbolt SSD).

    Maybe a backup, clean install, and restore may work, but would be quite a pain. BTW, late 2013 27″ iMac, so system compatibility not an issue.

  • I came on just to ask if there would be any foreseeable reasons this would NOT work on an SSD card. In the meantime though, I’m going to hold off on even trying that until I see if this .vmdk file issue gets sussed out!

  • Having same problem as Pete. the .vmdk on my HOME folder is not selectable.

  • noticed that when browsing from the virtual machine software to find the bootcamp.vmdk file the home folder is named root and when selecting that location the .vmdk is not visible yet when I look at the home folder in a std mac finder window then I see the file.

  • this command ran well:
    sudo VBoxManage internalcommands createrawvmdk -filename “bootcamp.vmdk” -rawdisk /dev/disk3

    but in the subsequent step 11 when it comes to identifying the bootcamp.vmdk in the home folder the file is visible but greyed out and not selectable in the virtual box interface. Any thoughts on cause or how to work around?

    Also the command line specifies a filename of bootcamp.vmdk however the subsequent lines say to look for a bootstrap.vmdk is this and error and should it be bootcamp.vsdk because I do not see a part of the process that creates a bootstrap.vmdk file

    • Hi Pete,
      The problem is usually caused by VirtualBox having insufficient privileges to access the file. This will happen if you launch VirtualBox directly from the Finder. Instead, use Terminal to launch VirtualBox using the command:
      sudo /Applications/

      This will launch VirtualBox with elevated privileges and should allow you to select the .vmdk file


    • For me this error (“bootcamp.vmdk” greyed out) was caused by including the quotation marks when copying and pasting Tom’s Terminal command. As a result, when Terminal created the file, it ends with .vmdk” rather than just .vmdk. Because the file extension is different VirtualBox doesn’t recognize it as a viable file. I removed the quotation marks from the beginning and end, and VirtualBox was able to see my file. A better solution is simply to delete the internal quotation marks at the time you’re copying and pasting Tom’s command into Terminal.

  • So this works on a Mac Pro 2009-2012. Because of the way we would be installing it?

  • Wondering if this can be done for XP, instead of a Windows 10 installation? I have some old games that I would love to run, such as CIV 2. I have the Bootcamp drivers still for XP and a licensed copy of XP on DVD.

    • I’d say give it a try, and let us know how it goes. Off the top of my head, the only problem that may arise is that the older Boot Camp drivers may not have drivers for recent Mac hardware. So there may be some issues with getting everything working on the Windows install. But you won’t know ’til you give it a whirl.


      • I have an external fire wire drive with bootable OS 10.6.8 and a Windows 7 partition.
        I upgraded to OS 11.3 on my internal SSD. The Mac OS on the external drive is still bootable. The windows partition is not. If I install the drive into the laptop (6.2) Everything works fine.

        1) can I make the external windows 7 partition boot?

        2) If I make a Wondows 7 partition on the SSD how can I import apps and settings from the external windows 7 partition to the internal windows partition?

        I have tried making a partition on my SSD and using winclone to bring over the partition from the external drive to the SSD, but I am unable to successfully clone the old partition to the SSD drive.