The subject says it all. I just installed FreeBSD on a leftover harddrive from a previous work laptop converted to an external 2.5-inch drive. Which means: No power cable's necessary either; it's a go just with a USB cable.
The performance is much better than I thought, almost the same as the speed of an internal harddrive. I'm lovin' it! ^^
Next project: Make another external harddrive (I have a bunch of them lying around) and try installing Windows XP on it, which would be useful to have as a “test” Windows installation. :D
BTW, the godly laptop that allows USB hard disk booting is: Dell Precision M65. ♥♥♥