joker, I think you are posting a few rumours. I have seen first hand, that a halo 2 save does not Ban you, I mean I have literally seen someone with many many H2 profiles saved on HD log on and off live multiple times over a period greater than 3 days.
Xboxgamer25, your experience mirrors mine exactly, I think they are checking hard drive though, because regardless, if you leave a modchip on, go on live, immediately log off again, switch off the mod chip and try and log on, you will find you are not banned. Whats happening is your machine gets read, for whatever details, it looks like HDD serial and eeprom at the moment, if they don't match or were banned previously then you are flagged and banned next time they do a sweep, which appears to be every 24 hours or less.
Has anyone tried
1. Getting banned
2. replacing HDD
3. Reflashing eeprom
I bet this works. and I bet this works if you have a halo 2 save on that new hdd. Just be careful because H2 always prompts you to sign into live.