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The use of propriety SSD’s on MacBooks limits the amount of space to dual boot Windows and MacOS on the same hard drive. Wouldn’t it be nice to boot windows from an inexpensive, fast USB 3.0 drive? Contrary to Apple and Microsoft, this can be done simply. This guide is not for external SSD drives! It’s for inexpensive, standard platter USB drives you can pick up at Bestbuy, Microcenter, Fry’s or preferable one you already have 🙂 I wrote this primarily because, up to now, I haven’t seen a guide that works for non-ssd based external USB drives.
This is very useful for backing up iPhones and iPads without using all the space on a SSD MacBook.

Jeremiah’s recipe for installing Windows 10 on a external USB hard drive:
External USB 3.0 Hard drive (100GB+) I used a 1GB WD Passort from Fry’s $59.99
Macbook 2012 or above
Copy of the amazing Winclone Standard $39.99
(http://twocanoes.com/products/mac/winclone) A download of Windows 10 from Microsoft
USB 8GB+ Flash Drive

Step 1:
Download the Windows 10 ISO on to your Macbook

Step 2:
Insert USB Flash Drive

Step 3:
Run the Apple boot camp wizard (Hit the magnify glass in the upper right corner and type “Boot Camp”

Step 4:
Run the boot camp installer, if prompted, point at the ISO download of Windows 10 in your Download directory.
This will partition your hard drive and take away space from OSX, this will be temporary. If need be, back up all your files to an external hard drive or the cloud to make room for Windows 10.

Choose the USB flash drive to create the Windows 10 installer with the Boot Camp drivers to allow Windows to run on Mac Hardware.

Step 5:
Allow Boot Camp to complete the installation; your Mac will reboot and install Windows.

Step 6:
The Windows installer will start, use the skip button on the Windows serial if you don’t have one available.

Step 7:
After windows installs, use “Start up” in the system preferences to boot to the Windows partition.

Step 8:
Allow the Boot Camp drivers to install on Windows

Step 9:
On the “Corona” search bar to the right of the start menu, type “regedit”

Step 10:
in regedit go to HKEYLOCALMACHINE > SYSTEM > HardwareConfig

Step 11:
Double click on BootDriverFlags and change “Value Data” to
“14” and click OK

Step 12:
Restart Windows and boot back into OSX

Step 13:
Back in OSX, plug in the USB hard drive you would like to run Windows from

Step 14:
Run WinClone, Choose BOOTCAMP on the left hand sidebar. Then on the right under “Destinations” choose the USB hard drive plugged in on Step 14.

Step 15:
Click “Restore to Volume”. This process will take some time. Once finished there will be 2 BOOTCAMP drives on the left sidebar. Close WinClone.

Step 16:
Back on the OSX Desktop, eject the BOOTCAMP hard drive and unplug it

Step 17:
Go back to the Apple boot camp wizard (Hit the magnify glass in the upper right corner and type “Boot Camp”)

Step 18:
Follow the wizard, this time choose “Remove Windows 7 or
later”. This will remove BOOTCAMP on the internal harddrive, freeing up space for OSX only

Step 19:
Plug the external USB drive back in your Mac, restart the Mac. On boot hold down the “option” key. This will show 2 drives, the first one will be for booting into OSX the other for Windows (Named “EFI Boot”). Choose the second drive.

Step 20:
You are now in Windows! Be careful not to unplug the drive until you shut down Windows.