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How to Install Windows 10 on an External Drive on a Mac

Windows 10 on a MacBook | Credit: Anthony Boyd


Things you need


  1. Your Mac
  2. At least 120GB HD or SSD (preferably USB 3.0 or higher)
  3. At least 8GB flash drive (also preferably USB 3.0 or higher)


  1. Windows 10 ISO
  2. Windows Support Software
  3. rEFInd

The Guide

Part 1: Obtaining Windows Support Software

  1. Open Boot Camp Assistant.
  2. Click Action on the menu bar then press Download Windows Support Software.
  3. It’ll ask you where you want to save a directory named WindowsSupport. Just save it in the Downloads folder for ease.
  4. Wait for it to finish downloading. Once done, you can close Boot Camp Assistant.

Part 2: Preparing the Windows 10 USB Installer

  1. Plug in your flash drive to your Mac.
  2. Open Disk Utility.
  3. Click View on the menu bar then press Show All Devices.
  4. Click on your flash drive under External over on the side panel. In my case, it’s SanDisk Ultra Fit Media.
  5. Click Erase on the toolbar.
  6. Set Name to Windows 10, Format to EXFAT, and Scheme to GUID Partition Map.
  7. Click on Erase to format the drive.
  8. Open up the Windows 10 ISO you’ve downloaded earlier. Drag all the contents of that ISO onto the flash drive we just fo.
  9. Open WindowsSupport, the drivers we download from the previous part. Drag all of the contents of that folder onto the flash drive.
Windows 10 USB Installer files

Part 3: Turning off System Integrity Protection

  1. Restart your Mac.
  2. Once you hear the boot chime, press and hold CMD + R to get into Recovery Mode.
  3. In the menu bar, click on Utilities then press Terminal.
  4. Execute csrutil disable.
  5. Turn off your Mac.

Part 4: Preparing your External Drive

  1. Unplug all USB devices except for the Windows 10 USB Installer and the external drive you want to install Windows on.
  2. Turn on your Mac.
  3. Once you hear the boot chime, press and hold ALT to access the boot menu.
  4. You’re gonna be presented with a list of bootable devices but we have to boot into the Windows 10 USB Installer we just created. Select the one labeled EFI Boot. You’ll see the Windows logo during boot up. If you see more than one device with that label, try each one until you find the right one.
  5. Once the boot up is successful, you’ll be greeted with the Windows 10 GUI installer.
  6. Press SHIFT + FN + F10 to open Command Prompt.
  7. Enter DISKPART. Then, execute LIST DISK. Identify the disk number of your external drive. In my case, it is 2.
  8. To select your external drive, enter SELECT DISK 2. Change 2 to your disk number.
  9. Once successfully selected, enter CLEAN. This will wipe the partition table and uninitialize your external drive.
  10. Now, we have to create a partition for Windows 10 to be installed in. Enter CREATE PARTITION PRIMARY SIZE=122880. This will create a 128GB logical volume. Of course, you can change it to whatever size you want. At the minimum, you should set it to 60GB.
  11. Enter ASSIGN LETTER=W. This will set the volume letter to W which will make it easier for us to reference the volume later.
  12. To format the partition, enter FORMAT QUICK FS=NTFS.
  13. Enter LIST VOLUME. We have to look for the EFI partition. It is formatted as FAT32 with Info set to Hidden.
  14. Select the volume by entering SELECT VOLUME 4. In my computer, the EFI partition has the volume number 4. Change it to the volume number of your EFI partition.
  15. Enter ASSIGN LETTER=Z.
  16. Verify that the partition you just created has its volume letter set to W and the EFI partition set to Z. Also check the volume letter of the Windows 10 USB Installer. In my case, it’s D.
  17. Enter EXIT to close DISKPART.

Part 5: Installing Windows 10 on your External Drive

  1. In Command Prompt, type in dism /Get-WimInfo /WimFile:D:\Sources\install.wim. This will list down the editions you can install. Identify the index number of the edition you want.
  2. To start installing, enter dism /Apply-Image /ImageFile:D:\Sources\install.wim /index:6 /ApplyDir:W:\. Change the volume letters if you have different ones. The index number I have is 6 for Windows 10 Pro. Change the index to your liking.
  3. Once it’s done, we have to verify if Windows was installed successfully in the partition. Enter DIR W: to list down the directories and files in that partition. Again, change the volume letter if you assigned a different one.
  4. To make your installation bootable, enter bcdboot W:\Windows /l en-us /s Z: /f UEFI. This will create the boot files in your EFI partition.
  5. Turn off your Mac and unplug the Windows 10 USB Installer.

Part 6: Installing rEFInd

rEFInd installation asking for root privileges
  1. Turn on your Mac.
  2. Once logged in, extract the rEFInd archive you’ve downloaded earlier. You’ll find a script with the name refind-install.
  3. Open Terminal and drag in refind-install onto the Terminal window. Press Enter to run the script.
  4. Enter your password when asked.
  5. If the output says that the installation is successful, restart your Mac.

Part 7: Setting up Windows

  1. Your Windows 10 installation should appear. Do note that it will always appear even if the external drive is unplugged. This is because we installed the boot files of Windows 10 onto the internal EFI partition.
  2. Select Windows to boot.
  3. Complete the setup.
  4. Plug in your Windows 10 USB Installer and open the BootCamp folder.
  5. Run setup.exe to install the drivers.
  6. Congratulations! We’re finally done!

Part 8: Turning on System Integrity Protection

  1. Restart your Mac.
  2. Once you hear the boot chime, press and hold CMD + R to get into Recovery Mode.
  3. Open up Terminal.
  4. Execute csrutil enable.
  5. Restart your Mac.

Other Notes

rEFInd with minimal theme | Credit: Anthony Boyd
List of disks in Terminal
  1. Open Terminal.
  2. Enter diskutil list. This will list down all of your disks. Take note of your EFI partition. In my case, it’s /dev/disk0s1.
  3. Enter sudo mkdir /Volumes/ESP. This creates a volume directory that the EFI partition can mount on.
  4. To mount the EFI partition, enter sudo mount -t msdos /dev/disk0s1 /Volumes/ESP. Voilà! EFI should appear in Finder.
  5. Open the volume named EFI. There you will find a folder also named EFI. Inside that, you’ll see a folder named refind.





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Eugenio Pastoral

I’m currently taking my degree on Computer Science with specialization in Network Engineering. I’m also a freelance photographer and designer.