Run Windows 10 from an external SSD disk? Yes, it works!

This is what I have done to make it work. The whole process takes approximately, 45 minutes but I think it is really worth it if you want to run a fast and clean installation of Windows 10 on your iMac.

What you need:

• An external SSD disk - Data will be erased ⚠
• An iMac 27" (Late 2013 in my case)

Temporarily, for the installation, you will also need:

• A running computer with Windows 7, 8 or 10 to copy the right files onto the SSD disk and make it bootable
• A running computer with OS X to download the BootCamp software
• A USB thumb drive - Data will be erased ⚠

First, in Windows, get yourself an ISO file of Windows 10. You can use the Media Creation Tool provided at https://www.microsoft.com/en-gb/software-download/windows10 and choose to save it as an ISO file (do not create a bootable USB media).

1. Double click on the .ISO file. This will mount the installation disk onto a drive (in my case, letter E).
2. Plug the SSD drive onto a spare USB port
3. Open a Administrator command prompt and type:
   1. diskpart
   2. list disk (identify the number of your external SSD drive)
   3. select disk 2 (or another number for your external SSD disk, be careful not to wipe your internal disk, usually on 0).
   4. clean - Be careful, if you have selected the wrong disk, it will erase all the data on it ⚠
   5. create part pri
   6. format quick
   7. assign letter=Y (or another letter if already used)
   8. active
   9. exit
   10. dism /apply-image /imagefile:"E:\sources\install.esd" /index:1 /ApplyDir:Y:\(change E to another letter for the mounted ISO file)
   11. BCDBOOT.EXE Y:\WINDOWS /S Y: /F BIOS (this will make the drive bootable in BIOS only (not EFI)
   12. exit
   13. Eject all drives cleanly or simply shut down your PC.

Secondly, on your Mac, open BootCamp assistant (click on Launchpad and type "bootcamp" to find it) and ONLY tick "Download the latest Windows support software from Apple". Save it all on your USB thumb drive. Shut down your iMac. Remove the USB thumb drive and keep it in your hand.

Power up your iMac by pressing the option key (alt). You will see an orange icon with Windows. Select this and wait until you see "Getting devices ready", wait a little bit longer until you hear the startup chime and press the option key (alt) again to make sure you boot on the external drive. That's it!

Now you have a copy of Windows running from your external USB drive. You just need to install the BootCamp drivers on your USB thumb drive (\Bootcamp\Setup.exe). Reboot.

That's it, you've got Windows 10 running natively on your beautiful iMac.

Enjoy!