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

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Mac O’Clock

9 min readJul 3, 2020

Windows 10 on a MacBook | Credit: Anthony Boyd

Hello! I guess this is my last resort. Unfortunately, I can’t get SASM to work natively on macOS Catalina for my computer architecture course. At this point, I can either work on Windows 10 or Ubuntu (or whatever Linux distro you can think of). As per the title, we’ll be installing Windows 10. There are a lot of ways to do this but this method provides more control over the installation process.

My method addresses the issue on storage. Most entry-level Macs only have 128GB of storage. And if you’re human, 70% of that is already filled with files. There’s really not much room there for Windows.

Boot Camp is Apple’s way of letting users get Windows on their devices. But… We wont be using that. I’m pretty sure you’ve already figured out why. Well if you haven’t, let me tell you. Boot Camp doesn’t allow you to install Windows on an external drive. So, we’ll be going around that restriction just like how you install Windows on normal PCs.

If you have extra storage, might as well use Boot Camp to avoid the hassle.

Overview

I’ve tested this on pre-UEFI and UEFI Macs, and it has been working great. Granted, you might have to do some additional steps for pre-UEFI systems but we won’t be doing that here. This guide is geared towards UEFI Macs which are models from 2013 and above.

GUI installers are pretty convenient, huh? Well, we won’t be using those in this guide. We’ll be using Terminal on macOS and Command Prompt on Windows. Over this whole process, you’ll learn how to manage drives using DISKPART, install Windows using DISM in command prompt, and how difficult it is to work around Apple’s kernel for Macs.

Well, the last one might be hard for you to realize because I already written this guide for you to follow. There’s not much headache on your side at this point.

Things you need

Before we begin, make sure you have these stuff.

Hardware

If you already are using an external drive that you wanna use for this, that’s fine. Just make a backup of your files from that drive first before proceeding because we have to reformat that drive.

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

Software

Obviously, you need the Windows 10 ISO, but that’s not the interesting part. By the way, this setup takes into consideration long-term use thus the need of installing a custom boot loader. It will allow us to choose which operating system we want to boot into without accessing Apple’s boot menu. You can really use any boot loader you want but I prefer rEFInd.

Windows 10 ISO
Windows Support Software
rEFInd

The Guide

Just a quick disclaimer before we begin. Do note that I’m not responsible if things go wrong. Proceed at your own risk. Make sure you have backed up all of your files!

Part 1: Obtaining Windows Support Software

For Windows 10 to work properly on our computers regardless of whether it’s a Mac or not, we need drivers. Apple consolidated all of the drivers we need for Windows. We can download them using Boot Camp Assistant.

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

Part 2: Preparing the Windows 10 USB Installer

In this part, you’ll be learning how to create a bootable Windows 10 USB Installer for Macs. We’ll be formatting the drive using Disk Utility.

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

Part 3: Turning off System Integrity Protection

This part is needed since we’re gonna install a custom boot loader. But, don’t worry! We’re gonna turn it back on after the installation.

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

Part 4: Preparing your External Drive

Oh, boy! Here comes the fun stuff. By far, this is the tedious part of the whole guide. We’ll be dabbling a bit with Command Prompt. Hopefully, you can learn some new concepts here! Be sure that you already have a backup of your files from the external drive.

Unplug all USB devices except for the Windows 10 USB Installer and the external drive you want to install Windows on.
Turn on your Mac.
Once you hear the boot chime, press and hold ALT to access the boot menu.
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.
Once the boot up is successful, you’ll be greeted with the Windows 10 GUI installer.
Press SHIFT + FN + F10 to open Command Prompt.
Enter DISKPART. Then, execute LIST DISK. Identify the disk number of your external drive. In my case, it is 2.
To select your external drive, enter SELECT DISK 2. Change 2 to your disk number.
Once successfully selected, enter CLEAN. This will wipe the partition table and uninitialize your external drive.
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.
Enter ASSIGN LETTER=W. This will set the volume letter to W which will make it easier for us to reference the volume later.
To format the partition, enter FORMAT QUICK FS=NTFS.
Enter LIST VOLUME. We have to look for the EFI partition. It is formatted as FAT32 with Info set to Hidden.
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.
Enter ASSIGN LETTER=Z.
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.
Enter EXIT to close DISKPART.

Part 5: Installing Windows 10 on your External Drive

Now that the partitioning is out of the way, we can begin with the installation. We’ll be installing Windows 10 via DISM. Essentially, we are deploying the Windows 10 image onto the partition we just created.

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.
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.
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.
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.
Turn off your Mac and unplug the Windows 10 USB Installer.

Part 6: Installing rEFInd

In this part, we’ll be installing a custom boot loader. rEFInd will make it easy for us to select operating systems on boot. Fortunately, the author created a script for us to use.

rEFInd installation asking for root privileges

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

Part 7: Setting up Windows

If it went well, you should’ve been greeted with the rEFInd boot loader.

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.
Select Windows to boot.
Complete the setup.
Plug in your Windows 10 USB Installer and open the BootCamp folder.
Run setup.exe to install the drivers.
Congratulations! We’re finally done!

Part 8: Turning on System Integrity Protection

This part is crucial and may leave your system vulnerable. But, don’t worry! We’ll turn it back on now.

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

Other Notes

I bet you’ve noticed that rEFInd looks ugly. Well, I have something to tell you. You can install themes. Check this one out that goes well with the aesthetic Apple is going for. rEFInd is very customizable but I’ll leave the configuration to you. You can find the documentation of the boot loader here. Look at how my triple boot setup is recognized by rEFInd below.

rEFInd with minimal theme | Credit: Anthony Boyd

You might’ve realized that if you have a larger drive, only one partition is present, your Windows 10 partition. This is because we haven’t created the partition for the unused space yet. To do so, just open up the Disk Management tool on Windows and create a partition for the unused space of your drive. Make sure you set the format of that partition to EXFAT so that you can have read and write capabilities for both macOS and Windows.

If you want to configure rEFInd to install themes or whatnot, you have to access the rEFInd folder in the EFI partition. To make things easier for you, here’s how to mount the partition and how to locate the rEFInd folder.

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

If you want to uninstall the boot loader, it’s relatively easy. Just mount the EFI partition and delete the folder named refind. If you want to remove Windows 10 from your external drive, that’s easy too. Just delete the partition from Disk Utility.

Lastly, I’ve had a couple of people try this on a 2018 MacBook Pro and it seemed to fail during the post-installation phase. An OOBE error keeps on occurring. The keyboard and mouse inputs weren’t recognized which prohibited us from working around the error. If you have any solutions regarding this or anything at all, feel free to leave a response.