There is always the possibility to run Windows 7 as a virtual image (e.g. VMWare, VirtualBox). Works well from an external drive and has the advantage that you can start the image from any computer that has the software installed.
Update 2 I tried to install Windows 7 on an external USB drive following the How-To in the first Update. Unfortuantely I didn't get it to work. Getting it copied to the external drive worked well but after rebooting I got an error about not finding this Windows installation.
I have followed BleepToBleep's instruction but couldn't get Windows 7 to boot on external hard drive. It supposed-to-be-bootable hard drive failed to start Windows installation after showing the Windows logo.
The key to getting the whole thing to work is to run part of bootcamp - the one which creates a Windows partition - but (and here's the key) to make that partition on your internal hard drive VERY small (<1GB).
Then you can run the Windows install from an external USB. BEFORE running the install, you need to enter CMD within the install utility to "flag" the external hard drive as active and bootable.
It will work for any Mac, but especially with retina and air users who are having trouble getting other methods to work. The link above is formatted more nicely, but I have pasted the text below. It will work if you follow each step iteratively.
Guide: create external Windows 7 boot drive for Macbook
Perhaps you want or need to install Windows on your Retina Macbook Pro. This is not hard to do, if you don’t mind the Windows partition taking up space on your disk drive -or- if you are willing to deal with the performance degradation from using the virtual machines: Parallels or VMware.
Instead of installing Windows on a separate partition within an internal storage disk, you can opt to buy an external Thunderbolt solid-state drive on which to place the Windows OS. You could just as easily use an external hard drive (instead of a solid-state drive) if you prefer a lower cost/GB. It just won’t be as fast.
What these look like:
This frees up your internal drive for all your Macintosh computing needs, while also enabling a large external disk on which to place a robust Windows install.
This guide is fairly detailed and only requires that you can follow a series of specific steps. If you can tie your shoes and you can read, you can probably run Windows off an external Thunderbolt drive.
A SUMMARY OF THE STEPS (so you know what you’re getting into)
It should go without saying, but make sure you have backed up your entire system before you proceed.
1) Resize your internal drive partition to make room for windows boot files WITHOUT using Bootcamp. Bootcamp uses a 20GB minimum and we don’t want to lose that much space.
2) Install Windows on external Thunderbolt drive.
3) Install Bootcamp Drivers
1) Create a FAT formatted partition within your MBP internal disk. I call this “the sliver” because it is so-so tiny.
you can use Disk Utility or Terminal to resize your MBP internal drive
A. to use Disk Utility
Open it from Applications»Utilities
select the Drive from the top of the hierarchy
click the “Partition” tab
Beneath the “Partition Layout” pane, select your Macintosh HD (or whatever you have labeled you system disk).
Mentally now, subtract 1.0GB from the current size of your internal drive. For example, your drive may read 250.14. In the “size” box, type 249.0 You can actually shrink it by as little as 400MB, but let’s leave a little buffer. This is going to allow you to create a new FAT partition into which Windows will automatically place boot files
now, click the new blank area below the main partition. Format it to MS-DOS (FAT)
Things should look something like this:
B. to use Terminal (faster, but less visual)
open terminal: Apps»Utilities
You need 2 pieces of information from the resulting table:
1) the size of your main partition and
2) your internal partition identifier (e.g. disk0s2) Double-check. But if you’re typing in terminal you already know that you should be precise anyway
now, execute the resize volume command:
diskutil resizeVolume disk0s2 249G MS-DOS FAT 1.1G
In place of “disk0s2,” insert the identifier for your internal system disk partition.
now you have an internal FAT32 “sliver.” Confirm this with diskutil list
Now, there is a SECOND step NOT to be missed: tagging this tiny internal partition as “Active” for the Windows install. Luckily, your Windows install USB/DVD comes with the utility. This is detailed in step 4, below.
2) Create (if you haven’t already) your USB Windows install disk. Go ahead and use Bootcamp utility for this.
Just make sure, in the first window, you deselect the last option for “create Bootcamp partition and install Windows.” We already did this.
So, select “Create Windows 7 or later … ” and “download the latest Windows support…”
Deselect the last option.
this will take a while.
Sometimes, the utility has trouble getting the support software from Apple (fails). This is fine. By the time it reaches this point, it has already created the boot disk. BUT, this means you have to download the Bootcamp drivers manually. You can download them here. Then copy the Bootcamp_version-whatever folder onto the Windows install USB stick. It has been labeled WININSTALL by Bootcamp.
3) Boot to Windows install. Restart your computer, holding the Opt key at the perfect moment! It can be tricky to time it right. Select to boot from the USB “windows” drive. It is orange.
4) Mark internal “sliver” as active. After the computer boots to the USB drive, click to select your language. BUT then we need to enter the “Repair utility” to set the small FAT sliver inside the computer to “active.” Don’t worry: you’re not actually performing any repairs. You’re just using the command-line utility that is built-in
After you hit “Repair,” go ahead and skip all the automatic options, so that you can use command-line.
In order now…
select disk # —in place of “#,” your internal disk listed
select partition # —in place of “#,” the sliver FAT partition we created
assign letter=a (or whatever. Get crazy. Call it “Z”)
5) Restart computer, hold option at the perfect moment. Select the USB Windows disk again (the orange one). Enter Windows install setup. You know, keep hitting “next, next next,” until…
6) Select a “Custom install,” when presented with the option, so you can select your external Thunderbolt disk to install. Do not install on your internal Mac drive. Do not install on the internal sliver partition.
Go ahead and format the whole drive using the Format button, after selecting your external Thunderbolt partition. Don’t format any other partitions
The format preserves existing partitions, BUT if you want to create others (perhaps to share with you Mac side) later, you can resize your primary NTFS partition within Windows after you install it.
7) Finish the Windows install. Each time it reboots, you will need to hold down the option key to choose to boot to the new Windows volume you have created during the install. Don’t boot to your USB stick anymore. The Thunderbolt Windows installation appears on the boot option screen as an internal disk, labeled “Windows.” If you miss the timing to initiate the boot menu, don’t worry. Your computer will boot into OSX. Just restart your computer and try, try again.
8) Install bootcamp drivers. After the installation completes and you log on for the first time, open your Bootcamp_version-whatever folder you stored in the WININSTALL USB drive you created in step 2.
Double-click the setup or autorun Application.
If your install did not work, look over the instructions again. Though this process does not require any special skill, it is easy to mess up a step. Don’t panic. You already have a complete backup of all your data right??!
Boot into Mac OSX, open Terminal, and type:
If your table does not look like mine, ask yourself, “Why self?”
This will not work with a bootleg version of Windows. Click here for an explanation.