The reason I want to boot from an external hardrive is that it's for a laptop. There's not much difference in the price of a 250GB external hardrive than a 60GB 2.5 internal hardive.
Plus I'm not sure if there's a problem with my laptop. Recently the hardrive failed and I had to replace it. Now the new hardrive is being accessed every second and is never completely quite even when the computer is idle. When I go into task manager the CPU never goes to 0% and usually fluctuates between 6% and 15% with nothing running in the background. So I don't know whats going on there.
So anyway I thought booting from an external drive may sort this problem, plus if it goes again I would stand a better chance of recovering data from an external hardrive by connecting it to another pc.
I can't just try it as I haven't bought an exernal hardrive yet. Thought I would get advice first!