My current theory.
When you go on XBL with an XBox MS marries your EEPROM to your HD serial in their database.
If at anytime in the future that marriage is broken (ie EEPROM or HD is changed, but not both) then the EEPROM is banned (and possibly the HD too!)(there is now a separate topic for this theory here http://forums.xbox-s...howtopic=304652 <-- GO HERE!!)...
A solution would thus be to swap both EEPROM and HD (bummer, but just put your big Xbox HD in your computer and get another big HD)
BUT, the problem is, does Halo2 take this further? Does H2 scan model numbers? If H2 scans model numbers and reports them to MS then they can also ban you if your HD model is not one they use in Xboxes... and this is the killer.
So, based on what I can see
1) if you are going to use an Xbox on Live and you want to upgrade its HD, Upgrade its HD *BEFORE* you take it on Live for the first time
2) currently rumours : if you have an upgraded HD *do not* go on live with H2!!!
2 needs to be tested unfortunately. Someone who has fine access with every other XBL game and as far as Live knows always had the same eeprom and HD (upgraded of course) needs to go on Live with H2 and see if they get banned... and if they do that sucks.
if 2 happens then we have 2 options
* buy a new xbox or downgrade back to stock
* get a new virgin (to XBL) eeprom (because the old eeprom and HD were married)
and everyone else has to either use an XBL stock Xbox, or a modded xbox with dual-drives, one with stock, one without, and always go on live with the stock drive, from the very beginning.
sucky poo.
(luckily, it appears "2" doesn't happen
)...
UPDATE:
There seem to be enough confirmations to say that my main theory is correct, and also that H2 is not banning people based on HD model numbers.
So, from what I can tell, you only get banned if you swap your HD *after* you first go onto XBL with a given Xbox.
If you're planning to upgrade an Xbox's HD, you need to do it before you first connect it to XBL.
But if you didn't do your upgrade before you signed on to XBL with that Xbox the first time, then you should sell your Xbox, and remod another one.
And finally, remember to use an XBL blocking BIOS so you don't get banned for having an active modchip!