I am using a 20gb harddrive right now and its packed full. A friend of mine upgraded to a 120gb and gave me his 20gb. So now I have two of these. Can I use the same xbox live account on both (meaning I'll switch them out depending on what games I'm playing, but use both on Live with the same account)

Only if you use a memory card to hold your profile.
And even then, not for everything...

But when I put in the other harddrive won't it act like a new unit (the harddrive was never used) and let me inport my account?

Yep which honestly is a waste of time. You will be transfer the profile and wait up to 10 minutes each time you want to swap back and forth to use like arcade games over and over. Then at some point you are using a memory card just for saved games because swapping out won't let you access the last saved game without the card. Too much drama. They say a 60 gig is coming out.

Yep which honestly is a waste of time. You will be transfer the profile and wait up to 10 minutes each time you want to swap back and forth to use like arcade games over and over. Then at some point you are using a memory card just for saved games because swapping out won't let you access the last saved game without the card. Too much drama. They say a 60 gig is coming out.

Exactly...nothing but a pain. You are better off getting the 120 GB drive and then once you transfer your stuff to the new drive then you sell the two 20GB drives.