Boot Windows On Your Mac From An External Hard Drive

You need aUSB 3 or Thunderbolt thumb drive or external hard drive

Slightly more difficult method that requires access to a PC running Windows to set up. Worse performance than Boot Camp, but better than most virtual machines. Requires you to restart your computer every time you want to run Windows, but doesn’t take up space on your Mac’s hard drive.

Don’t have a lot of free space on your Mac’s hard drive? It’s also possible to boot a full version of Windows on your Mac from an external drive. You can use this disk on any Mac, and all of your installed Windows programs will come with you.

The catch? You can’t use Boot Camp to set the process up. I’ve found a method that works, but it requires access to a PC running Windows. Ask a Windows-using friend nicely if need be – you’ll only need the machine once.

You’ll also need a free program called WinToUSB, and a USB drive to install Windows onto. I used an old external hard drive, but if you have access to a flash drive bigger than 32 GB it should work. USB 3.0 is recommended.

Setting Up

Plug your USB drive into the PC, then open the Disk Management utility. Right click the USB disk itself (making absolutely certain that it is your USB disk – you’re about to delete all files on the drive).

If you see the option to “Convert to GPT”, click it. This will format the drive, but is necessary in order to create a drive that will boot on your Mac (or any UEFI-compatible device). If you see the option to “Convert to MBR”, don’t click it: simply delete the partitions on the drive.

Double-click the empty space on your newly empty drive and create a FAT32 partition – it doesn’t need to be bigger than 100 MB – to serve as the boot sector. Next, create an NTFS partition for the remaining space – this is where Windows itself will be installed.

Your external drive is now ready for WinToUSB, so fire that application up. You’ll need to point it toward your Windows 8 install disk/ISO, then to your USB drive. Assign the Boot and System partitions you created earlier.

Click next, and WinToUSB will install Windows on your USB drive. When it’s done, unmount the disk from the PC and plug it into your Mac. Shut your Mac down, if it’s currently running, then turn it on while holding the Option key.

You should see your USB drive as an bootable option – click it to continue, and Windows will start (though it may restart once or twice to complete the installation process). You’ll have Windows running, but your WiFi and a number of other things won’t work – you need drivers.

Reboot your Mac to OS X and download the Boot Camp drivers. You’ll probably want to put them on another USB key, so you can access them from within Windows. Reboot into Windows, then run the installer.

Eventually you’ll have a full version of Windows running from USB, complete with Mac drivers. A huge advantage of this set up is you can run the same instance of Windows on any Mac, and all your programs will come with you.

References

http://www.easyuefi.com/wintousb/

https://discussions.apple.com/thread/5431182?start=30&tstart=0