MacBook – Install Windows 7 on external drive and boot it from a Mac

As far as I know Microsoft lock their Operating Systems so that you cannot run them directly from an external storage device. Microsoft say something about it not being a 'primary means of storage.' More like them making an attempt at stopping people running multiple computers of on copy of the OS, as allowing them to install it on any storage would allow people to make loads of copies. Hence, Linux does not have this problem. I personally do not do much gaming, and have never needed Windows on my MacBook Pro, so have not dabbled in the whole 'Windows on my Mac' thing. If you really do have a problem with sticking Windows on the MBP's Hard drive and booting either natively or through virtualisation, then the following links may help, although the guides are designed for XP and I have no idea if they will work on 7.

Prepping the install CD (scroll down)

Installing

As I said, I cannot guarantee this will work for 7. I hope, however that I have helped in some way.

Yes, I just got a new Mid-2012 MacBook Pro 13" and did this very thing over USB 3.0. I know of no reason why it would not work on a MacBook Air.

When booting from an external spinning hard drive over USB 3.0 on my new MacBook Pro, the external hard drive sends data back and forth to the Mac about as fast as it would if it were installed internally. I have not specifically measured drive speeds in this scenario; I'm just saying that it performs quite well.

As far as I know, all Macs with USB have been capable of booting from an external drive connected via USB--all the way back to the first iMac circa 1998, which only made use of USB 1.0. Booting from an external USB 2.0 drive, whether a flash/thumb drive or a spinning hard drive, is, however, quite slow.

When you boot your Mac, immediately after the startup chime, hold down the Option key. Your Mac will show you a row of icons of each volume or partition currently connected to the Mac (internally or externally) that holds a Mac OS X installation that you can successfully boot from. Use the arrow keys to select which one you want to boot from, and hit the Return key to boot from that volume.