I may well be be wrong but I'm fairly sure you can't boot from external USB discs. You could probably install a virtual machine on an external drive and run it that way.
More importantly, why?
Bandwidth is crap though
I wanted to be able to take the HD away easily when needed ..
I wanted to be able to take the HD away easily when needed ..
This.
If your Mac has a DVD drive, junk it for an additional HDD.
Or, partition your current and bootcamp it.
Agree with the above. Bootcamp is a pain. People who actually need to switch between operating systems with any degree of regularity, will soon find it annoying and cumbersome having to reboot. Those that just need Windows would be better off with a normal laptop. Arguably, I suppose.
Having Windows and Mac separated only by a four finger swipe is the way of Kings. Copy & paste between operating systems is magic.
Although yes, plenty of power is a necessity. 16GB is fully for the win.