Q: How to run Boot Camp on External Hard Drive?

Generally Speaking, no, it is not possible.

Boot Camp does not support booting from an external drive in any system, especially not in a notebook.  You could try to use a different EFI boot loader (like rEFIt, or rEFInd) but then you will run into the next major hurdle.  Until very recently, Microsoft did not support booting any version of Windows from a removable media.  Yes, you could build your own custom installers and make it work, but it was not supported.  Now recently in WIndows 8 or 8.1, Microsoft has added some support for booting from external drives, but it is a difficult task to accomplish from what I understand.

About the best bet I can see for you would be to buy a Thunderbolt external drive like the Buffalo Ministation (available form Tiger Direct for under $200 USD for a 1TB drive with USB3 and Thunderbolt cables).  I believe that you can easily install MacOS on one of those drives and boot/run your Mac from there.  I don't think it would allow you to boot Windows from there, but you could still install your Windows programs on a partition on that drive and access them when booted from the internal drive.

My suggestion was that you run MacOS from the external drive, and run Windows from the internal drive, so you should not have any problems running Windows games (when you are running Windows).

Depending upon the games you want to play, you might be able to use virtualization and run Windows in a virtual machine while still running MacOS.  If you do this, you could then store your virtual machines on the external drive.

As I said initially, it is not possible using Boot Camp.  There are other tools which might help you, and I mentioned those in my first reply as well.

I can report two successes running Windows natively from external hard drives. So what you want is technically possible.

Success 1: I was able to configure Windows 7 to work from external thunderbolt drive (SSD) on MacBook Air mid 2011.

Success 2: I was able to configure Windows 8.1 to work from external thunderbolt drive (2 tb hdd) on MacBook Pro Retina.

In both cases no significant space was uses on the actual internal SSD.

Keep in mind though, MacOs Bootcamp Assistant application just does not implement the needed functionality, so you can't do it via it. If you (or somebody else) is interested in details, I can try to recall and write down the details of what I've done.

I can write down hardware that I have used since I remember it very well.

For thunderbolt drive I have used Seagate GoFlex Thunderbolt Adapter with either Ocz Vertex 3 480 GB SSD or WD Green 2 tb 2.5 inch drive. Worked reliably and easy to assemble since adapter provides a standard SATA connector. The only catch is you might want some "duck tape" to lock the non-standard hdd in place. Or you can go for compatible Seagate 1 TB HDD drive.

Warning: I have tried Samsung 840 Pro with Seagate Thunderbolt Adapter, but it was unstable. Probably not enough electric power supplied via thunderbolt. Or may be because 6gbit SATA was too fast for adapter and it was unstable. I haven't bothered to investigate deeper - I've just used Samsung 840 Pro in a completely different machine as an internal drive.

As far as thunderbolt cables are concerned, both short and long ( 1.5 m) ones worked well.

I suspect Seagate Desktop Thunderbolt Adapter or any other thunderbolt drive will work equally well.

For the MacBook Pro Retina and Windows 8.1 the setup procedure was the following:

Step -1: Make sure your Thunderbot drive works, stable and reliable.

Step 0. Make sure you have a complete Time Machine backup of you Mac OS hard drive. Seriously. Very easy to mess things up completely.

Step 1. Get a Windows 8.1 64 bit iso. You might want to try it out first before committing money to it. For that you can download an official 90 day Windows 8.1 Enterprise evaluation from Microsoft. It's somewhat hidden from mainstream experience, but search engines help you there.

It is important to use 64 bit, as 32 bit will not work.

Step 2. Get a USB flash drive. 8 GB USB flash drive worked fine for me.

Step 3. On Window box or from virtualized Windows machine make a bootable setup USB. I have used Rufus for that. Make sure you have selected "GTP partition scheme for UEFI computer" and FAT32.

I've tried MBR partition scheme, but it hasn't worked for me.

Step 4. Once bootable setup USB is created, shutdown everything. Plugin your external thunderbolt drive and bootable setup USB flash drive. Hold ALT (Option key) to choose from what thing to boot.

Step 5. If you are lucky you will see yellow "EFI Boot". Yellow usually meens 'external'. That meens you are going to boot into setup from USB flash drive. Go for it.

Step 6. In 5-10 minutes you should get into Window setup. Whenever you are asked, always choose "Custom" or so, till you get to the hard drive selection screen. You should see list of many partitions coming from 2 drives - one internal and one external.

WARNING: Be super careful here!!!!!

Step 7. Convince Windows to install itself on the drive that is external. You might have to delete all of the partitions of the external drive and leave it completely unpartitioned. This will make Windows autoconvert it's partition scheme to GTP.

WARNING: Think twice before deleting any partitions; do not delete any partitions on your internal drive.

Step 8. Let Windows install itself. I recommend turning off windows autoupdate for now, especially drivers part of it, since Windows 8.1 autoudate seams to be pushing buggy drivers at the moment.

Step 9. If you are SUPER lucky after reboot (and holding ALT/Option) you should see gray "EFI boot" that will boot Windows from external drive.

Step 10. Download and install Apple latest bootcamp drivers, things should work normally once those drivers are installed.

Step 11. Enjoy!

For the external Windows 7 bootcamp on MacBook Air 2011 I've followed a convoluted road, as ot was a result of a gradual migration. I suspect a shorter road will work.

First, I've installed a second SSD drive on MacBook Pro 2012 early. This involved moving original Apple SSD to dvdvrom bay and MacOs installation with it. And inserting Samsung 840 Pro in the original HDD location.

Second, I've used a combination of Bootcamp assistant and Winclone to get Windows installed on that Samsung 840 Pro  and occupy the whole drive. So I had 1 MacOs and 1 windows drive.

Third, I've cloned that dedicated windows drive into external OCZ Vertex 3 drive.

Forth, I've place OCZ Vertex 3 drive into the Seagate Thunderbolt Adaper, connected MacBook Air to it, booted it using ALT and luckily for me things worked!

Fifth, reinstalled Apple Bootcamp drivers to solve the device differences.

Unfortunately my memory is somewhat fuzzy, as it was a long time ago, so sorry for many missing details.

SUMMARY

I believe the key experimental finding there is that if you are using Thunderbolt drive, you can do it (once way or another). But you need to have time and dedication to get the partitioning scheme right. And have some luck (as different Apple hardware models seam to have slightly different boot preferences).

I've never had success in running Windows from external USB drive (once got very close to actually booting it, but it was blue-screening in disk.sys).

Your advice is ...well....not good. First off, no games will play in a virtual machine unless it's solitaire. Second, you can boot Windows from a Thunderbolt drive or even some USB 3.0 drives assuming they're the correct chipset. The catch with most USB 3.0 drives is that they tax the CPU when running on OS from them. You'll want a high quality SSD either way, but do research before proceeding. I have installed Windows on intel macs natively, with 0 bootcamp assistance whatsoever. You just need the bootcamp driver iso from Apple and you're set. Please stop posting incorrect information, sir.

In case you are still trying to do this, yes it's very possible, I've done it myself.

Get a Seagate Backup Plus external thunderbolt sled adapter ($95 or so on Amazon).

Get a good SSD from Samsung or Sandisk ($95-105 for a 256GB on Amazon).

Get a thunderbolt cable ($30 give or take).

Windows will boot from thunderbolt and run incredibly fast, and yes you can run dx11 games at full resolution with settings maxed out. I've had 0 issues on my 2013 27" iMac i5 with the Gtx 775m 2gb GPU

How are you a level anything with poopy advice and information like this?

CrankTurner wrote:

 

Your advice is ...well....not good. First off, no games will play in a virtual machine unless it's solitaire.

Sure they will, just not well in many cases.

CrankTurner wrote:

 

Second, you can boot Windows from a Thunderbolt drive or even some USB 3.0 drives assuming they're the correct chipset.

How do you get Boot Camp to create the partition on an external drive?

Just not very well, indeed!

Honestly I never use Bootcamp, I have always just used the Bootcamp driver package for whichever particular Mac I'm installing Windows on, after the Windows install completes.

I just boot from a Windows 7 image that I installed on an 8GB USB flash drive (hold down the option key after you power on the mac), and install it on a partition I already created on the SSD from the disk utility in Mac OS. You can create it in exFAT format or just FAT, and then tell the Windows installer to reformat in NTFS when you're doing the install. After Windows is installed, it will come up as bootable option in the boot menu when you hold down the option key while powering up.