Can I use my Intel-based MacBook to boot into Windows 7 from an external HDD?

Question

Okay, so I need to use Windows for some simple applications. Problem is, I don't have a Windows desktop anymore and while I have both Windows and Linux partitions along with Mountain Lion on my installed hard drive, I'm probably going to wipe said drive and just use the entire partition for Mountain Lion. That said, I'd like to use an extra hard drive I have lying around to boot Windows.
My questions are:
1. Does my MacBook support USB boot to Windows?
2. If my laptop does support this, can someone point me to a set of good reliable instructions to install Windows onto the external hard drive and boot into it by USB?
Thank you; have a blessed day!

Answer 1

You seem to have a triple boot system that was done using the 10.6 installer disk and a copy of rEFIt.
I know because I had the same thing once.
I can tell you what will happen if you decide to upgrade 10.7 or 10.8
1: The EFI and GUID is going to change, rEFIt hasn't been updated in ages since 10.7 started rolling out back in about June 2011. There will be adverse effects with rEFIt so I hear.
2: The 10.8 upgrader is going to barf at your triple boot system, it will upgrade the 10.6 partition but warn you that the 10.8 RecoveryHD partition (10.7 and 10.8 both install) that it normally installs at the very bottom of the drive will not be installed and you can't enable Filevault.
Intel Mac's can have only 4 bootable partitions on the drive, the 10.7+ installer is normally dealing with EFI, MacintoshHD, BOOTCAMP, RecoveryHD. It barfs at anything else it doesn't understand and will not install RecoveryHD partition, which on a non-Internet Recovery capable hardware is required to reinstall the currently upgraded OS X operating system version. But you can always go back to the 10.6 disks and wipe and restore that way or via 10.6 USB boot key.
http://www.maciverse.com/install-os-x-snow-leopard-from-usb-flash-drive.html
So with 10.7+ you'll have to use something like Carbon Copy Cloner for the OS X partition.
So since rEFIt is going to be hosed, so will your ability to use the EFI based program to boot into the three operating systems at boot time.
I assume you found this out and why you want to dump Windows on a external drive. However Apple's EFI doesn't recognize Linux as a boot option.
There is a rEFIt fork that will allow Linux to boot on a Mac: http://www.rodsbooks.com/ubuntu-efi/index.html
Windows as you now know can't be booted off a external drive, however if your use of it is minor, you may want to consider using a virtual machine to run it in OS X or Linux instead using the free VirtualBox.
Windows in BootCamp or Virtual Machine?
You should Carbon Copy Cloner the OS X 10.6 partition to a external GUID OS X Extended J (JHFS+) drive.
You can use WinClone 3 in OS X to clone the Windows partition to a external drive, but it's a backup only, no booting capability. Restoring it will boot on the internal drive of course.
Linux you should already know how to clone it to a external drive using dd.
I advise you use the 10.6 install disks to set up your new partition scheme, but your going to have to uninstall rEFIt and return the machine and firmware back to basic Apple factory system as possible before doing anything.
EFI doesn't get erased, it resides in NVRAM and the EFI partition, controls the at boot key commands to boot off OS X install disks, so it's important to uninstall rEFIt completely before upgrading OS X.
If the Windows version your using is XP, to install that first using BootCamp in a fresh 10.6 install and reverse clone using WinClone before upgrading to 10.7 or above. It will be grandfathered in, but can't be reinstalled later.