[quote name='grundy']
I'm sure this has been asked and answered numerous times, but if I take my 250GB HD from my slim 360 and place it in my friend's slim 360, will I still be able to access my gamertag, rock band song downloads, game demos, XBLA games, etc.?
Basically, if we wanted to play RB3, would I be able to play all of my songs?
Who would it sign in for XBOX Live? She has a Gold membership, but is using a formatted external HD. Would I have to disconnect that before inserting my HD?[/QUOTE]
The DLC is tagged to the account and the console that the DLC was purchased on. It's not tagged to the hard drive.
So for your DLC to work on your friend's console, you would have to be logged into Xbox Live. This works fine if you're doing multiplayer (your gamertag and her gamertag profiles are logged in). But if she wanted to do single player, she would have to play under your gamertag for the DLC to work.
If you want no problems, it'd be easier to just bring your console over. That way, it doesn't matter whether you have an Internet connection or who's gamertag is logged in.