Booting Windows on a Macbook Pro with an external SSD
With the latest generations of Apple's Macbook's you are stuck to the amount of storage you have when you purchased it. This means that later on when you would like to add a second OS (like Windows for example), chances are that the space on your SSD is too limited.
I also had that issue. When I bought my MacBook Pro I thought I would only need 128GB of storage since I wouldn't be storing any movies or music on it. But when I started my second education I needed to have Windows on my laptop for software purposes *sigh*.
At first I thought of BootCamp for a Dual boot on my internal SSD, but that didn't turn out well. I would only have about 45GB of storage for the Windows partition. So that was a failure and I needed to find an other sollution.
The other possibility was that I would install the windows OS on an external storage device and boot from that.
The storage Device
The first question that was very important, was the storage device that I would use to boot Windows from.
First of all it had to be a fast device so I could boot fast and my programs would load fast.
Therefor my best shot was to put it all on a SSD. But which one? After some research I came up with a device that was performant as well as within the reach of my wallet
The StoreJet 500 from Transcend turned out to be the one to go for. It costs around €200 for the 256GB model and it is able to connect with Thunderbolt so I have some spare USB ports. Here you can find a nice review on the device.
For the installation you're gonna need a few things. First of all you'll need a working Windows machine or virtual machine running Windows. Make sure the machine is a 64-bit OS otherwise this will not work. Next to that you will also need a usb drive that has minimum 8GB of storage (I'm not sure if 4GB would do, because I used a 16GB drive) to store the Windows drivers on.
For all the steps you need to take in order to put Windows on the portable device I'm going to refer you to a very detailled article. There they explain what software you need and show step by step what you need to do in order to get Windows in your portable SSD(or HDD).
If you have a MacBook which is already a little older, you might wanna check out the rEFIt project. These guys provide you with a sollution for older MacBooks to be able to boot a non-EFI OS like Windows. I didn't need this because the latest generations apparently doesn't have that problem. But try it out first without the rEFIt.
Once you've managed to boot from the portable SSD(with or without the rEFIt), the fun starts. Go ahead and install Windows like you would do it on a brand new PC and there you have it: you can run Windows from a portable storage device.
But wait! we're not done yet! The last thing you still need to do is install the Windows drivers for your MacBook. The best and easiest way you can do this is by booting back into OS X and to launch Boot Camp Assistant. Once in there you only need to check "Download the latest Windows support software from Apple". This way you'll only install your needed drivers for windows on your USB drive.
After this there is just one thing left to do, install the drivers! So go ahead and boot back into you Windows and open the setup which is located on you stick under /BootCamp/Setup.exe.
End result and conclusion
And there it is, the result of all this work is that you are able to boot Windows from an external storage device from your Mac.
Ok, I must admit. If I'd known that I would need Windows so hard on my MacBook I would probably have invested some more money when I bought it in the first place. But then again if I had done so I would have spend a LOT more money than I've spend right now. The difference in price for a 128GB model vs a 512GB model is about €500. Considering that the external SSD "only" costs around €200. I actually didn't lose that much. The only disadvantage of this sollution I could think of is that your mobility is reduced a lot...
So to sum it up, if you're looking for a cheap and fast solution to your Windows problem and you're willing to give up some mobility, then this is your way to go. If you really dont want to constantly walk around with an external storage device, then you should've bought a bigger SSD in the beginning.