1) Clean the disk.
2) Playing halo 2 off the HD will get you banned from Live: first, the see that you have a different/patched BIOS loaded (M$ doesn't want that
), and then, they'll see your playing it off a the HD (which shows that you have a modded Xbox).
3) Use Xlink Kai or XBConnect. they're free.
and yes, you can play it off an HD. Just make a "games" folder in the E:\ or F:\ drive, then make a "halo 2 folder in the games folder, then place all of the files into the Halo 2 folder. Also, delete every .xbe file, except default.xbe