I am planning to buy an external HDD, the WD Passport 500GB with USB 2.0 & 3.0. The idea of having my files and programs and games' setups in my pocket seems nice (better than carrying a CD bag :P) anyways I thought of installing a windows OS on the HDD so that I can boot my windows setup on multiple pc's wherever I go. Some considerations:

- Is the External HDD recognized as a removable memory or a partition?

- First, the order of booting, although it is just few clicks, it might be impossible in some cases (My class's PC that is connected to a projector only gives a display on logon screen)

- Compatibility of the OS, Windows 7 can recognize many drivers till now so it wouldn't be a problem, but it needs some specs that are not available everywhere. If I want to put XP, then it will work on somehow all computers in here as of specs and needed RAM, but the drivers would be a problem.

- I read about using VirtualBox, is it practical?

I just don't use a USB drive since it is the most popular way of infection I know, So using my external HDD as a removable memory is something I wouldn't do unless if there is a way to install a portable AntiVirus or something similar.

10x in advance for any reply
there are many other ways to protect your HDD from viruses, running a whole freaking OS on it is not the most practical of solutions
spend some time on google you WILL find your answer
Zusynoid-x wrotethere are many other ways to protect your HDD from viruses, running a whole freaking OS on it is not the most practical of solutions
spend some time on google you WILL find your answer
I would be crazy to put an entire OS on it just to protect it from viruses, but what I want from an OS is like a portable computer in my pocket, (all my programs, files, pics all running and installed on an HDD system)

EDIT: It seems impossible to protect an external HDD from a virus, the problem is I need it to be plugged in viral and infected computers, especially for project presenting in school.
i advice you to run Ubuntu on it
that + an encrypted HDD would be as secure as possible
as to the window specific applications, you can either use virtualbox within ubuntu or use Wine
Installing an OS on the external hard drive will work, but only using the computer that the OS was installed from.
On any other computer it will not work.

their is a small application for ubuntu, that allows the hard drive to become bootable. and you can run a live boot from it using any computer. "forgot the name of the app"
use ubunto linux and ull be set. no viruses. very lightweight, and very powerful. windows's registry is not flexible enough to always work on several cases
Yes ! Exactly, Unetbootin ! :)
Thanks Zusynoid-x.
check : www.pendrivelinux.com they even provide a free multiboot or a normal boot + you can make the USB persistent. And you can install multiple OS on an external HDD.
Everything is explain for most Linux Distro. I used it couple of times for multiboot on my HDD and my Flash memory. I installed like Backtrack/Helix/Ubuntu and some security/tools Live Cd like Hiren's.
How about write protecting the external hard disk?
Well what about you set a strong encryption to the folders in your HDD ? You can use a software such as Winsesame http://www.aragonsoft.com/.
Google a bit you'll find other tools. Then the Data in the password protected folders can't be accessed unless by the owner that set up the pass, neither an unauthorized person or a virus can access them.
How about running a small footprint OS like puppy linux? I did it with a usb flash drive that installs winxp and runs puppy when needed. Also added to it a winpe and dsl linux, all in a grub boot loader. ONLY Problem was the file structure being visible to windows, which would allow a virus to embed itself to readily available files. That alone makes it unpleasant.
4 years later
UPDATE:
LiLu or Yumi?
any of those will get your USB ready to boot live linux from an image file.
Persistence now is added to save your progress as you go.
Other than DSL linux, ubuntu, Puppy and other assortments of linux distros, now you can get Mint linux with persistence.
Mint linux is a branch-off from ubuntu linux which is in its own right a debian distribution.

USB3 can make the experience quite pleasant.
BIOS setting is your only hinder against smooth sailing.
I found some laptops don't easily allow you to boot linux from USB as some settings are needed to set those up with your current windows installed. Very unpleasant failures from my part.
Laptops with muscle only seem to handle these option without incidence.
My PC won't allow these options as well since it is a Core2 Quad with DX3 intel board. Intended for windows environment to carry a punch rather than allow a complete open source experience.