So I'm going to my uncles for the rest of the week and my cousins have an Xbox 360. I usually stay up longer than the rest of them, so I have time to kill. I was planning on bringing some of my games and my hard drive.

So if I switch their hard drive with mine, will my account be on there?
and if I happen to get an achievement, will it save?
So I dont have to worry about recovering my gamertag?
(I wont be going online or anything, just playing campaigns and stuff like that)
Nope. The 360 saves all of it's information on the HD, so when you move it everything you have and everything you unlock transfers nice and easy. Your gamertag and profile will be fine.