Installing Windows 10 on an External USB 3 Hard Drive for Boot Camp

Submitted on Jan 01, 2016, 9:22 a.m.

**UPDATE**: Aug 05 2016: Surprisingly, the Windows 10 Anniversary Update (Version 1607) installed on this installation okay, although only via the VM (running under VMWare Fusion). It would NOT install when running from the bare-metal installation (option boot from EFI partition). Trying to install from the EFI booted partition results in the 'Cannot upgrade due to unsupported disk layout for UEFI firmware' error. Once the update was complete on the VM guest, it also booted fine from the EFI partition. I have no idea how, or why this worked. ;-)


A success story is a nice way to start the New Year. The internal drive on my MacBook Pro (Retina, 15-inch, Late 2013) has been getting a little full, and the Boot Camp partition was taking up about 45GB of space. I also wanted to run a proper Windows machine (including developer tools) and 45GB wasn't really enough to do much. And so began my quest to see if I could achieve the following:

  1. Install Windows 10 Pro (build 1511) on an external USB 3 SSD hard drive.
  2. Have my MBP recognize this drive as a boot option when pressing the option key at boot (turning my MBP into a Windows computer).
  3. Have VMWare Fusion recognize the external Windows installation as a valid Boot Camp source, allowing Fusion to run the _same_ Windows installation as a virtual machine while in Mac OS X.

I did a _lot_ of reading.

Here's a link to an almost workable solution using Winclone. I've read pretty good things about Winclone however, this route failed. The method involves allowing Boot Camp Assistant to create both the installation media, partition, and a working Windows installation on your internal drive, and then using Winclone to clone the Boot Camp volume to an external disk. In my case this resulted in the blue screen error of INACCESSIBLE_BOOT_DEVICE when trying to boot from the external drive in normal mode (although the installation _would_ boot in safe mode). I searched the Winclone forums, and read most of their help docs - most of which ended with "some USB 3 disk enclosures work, and others don't" and that was it. :-(

I then discovered this post - and well, this is the business. There's a bit of setup required along with one prerequisite - you need an existing Windows computer or Windows VM. I believe anything from Windows 7 upwards will would work since this machine is only going to be used to prepare the external hard drive and place the Windows installation files on it. I still have my very old but trusty ThinkPad T61p with Windows 10 installed, and so I used this as my prep machine. The general sequence I followed was this:

  1. Download a Windows 10 ISO image (version 1511 as of writing) from here. The ISO will contain options for both Windows 10 and Windows 10 Pro.
  2. 2. Use Boot Camp Assistant to create Windows installation media (using the downloaded ISO from 1. above) on an external USB 3 thumb drive (the current option says 'Create a Windows 7 or later version install disk'). This is the only thing I used Boot Camp Assistant for as it's a convenient way to create the required Windows installation media as well as download the Boot Camp Windows drivers (they'll appear in a BootCamp folder in the root of the thumb drive).
  3. 3. Download the `imagex.exe` application. You can download the entire Windows AIK - and yes - The Windows® Automated Installation Kit (AIK) for Windows® 7 is the correct version even though we're setting up a Windows 10 installation. There is also a link from the good post to a small application that will let you selectively download WAIK tools. Here's the WAIK forum thread and download link. I took a chance and used this to download just the Windows 7 AIK option - and it worked fine. Be sure to scan the download for malware. At the time of writing it was fine.
  4. 4. Carefully follow all of the instructions from the good post. In the unlikely event that Stephen's excellent post is not available, here's a link to the step-by-step section in PDF format.

And it all worked wonderfully. The first option-boot on my MBP to the external drive automatically started the Windows installation and it all completed fine. I now have Windows 10 Pro on a 480GB external USB 3 SSD drive as my 'Windows machine'. When option-booting the MBP - choose the EFI boot volume. To add this Windows installation to VMWare Fusion, choose New Boot Camp from the Add New Virtual Machine menu, or if it doesn't appear there, just choose 'New' and then install from Boot Camp, and point to the EFI volume of the attached external drive (note that the EFI volume may appear as 'untitled' when you've completed the steps above - just rename it to EFI).

Happy New Year.