The same thing happened to me, and here is what worked for me:
First, delete everything on ur hard drive that u added in xplorer by formatting the drive again in the xbox 360. Then, at the main menu, create a user profile, and just make sure that you save it onto your hard drive. It doesn't have to be fancy or anything like that, just make a profile like your friend would if you guys were gonna play spit screen multiplayer or something. Then, after you've created the profile, plug the HDD back into your computer, and then transfer all your stuff over in xplorer360. Once that's done, then plug it back into the Xbox, and your profile should show up. If that doesnt work, then I'd suggest trying the "recover your gamertag" option.
This should not be a complicated step by any stretch of the imagination.
This post has been edited by trees415: Apr 1 2010, 05:11 AM