(This theory was originally discussed in great detail and was almost totally verified in this thread http://forums.xbox-s...dpost&p=1984331, but following that, most of the discussion and verification took place in this thread. The "Marriage Theory 101" thread http://forums.xbox-s...howtopic=335984 makes a good introduction to Marriage Theory )The Marriage 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!)...
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)
Further, there is a belief that MS is flagging GamerTags which have been banned, and if this tag is used on an XBox with a non-stock HD, then the XBox will be banned (Both EEPROM and HD). So If you get banned you need to replace your EEPROM and HD with EEPROMS and HDs which XBL has never seen, as well as replace your GamerTag if you want to use a big HD.
(confirmed by MS tech support : http://forums.xbox-s...0)(slow xbl logins with flagged GTs : http://forums.xbox-s...howtopic=306498)(detailed explanation : http://forums.xbox-s...dpost&p=1963940)(detailed tests : http://forums.xbox-s...howtopic=308731)(ydgmms' confirmation : http://forums.xbox-s...dpost&p=2038318)(MS confirmation of stock xbox is fine : http://forums.xbox-s...howtopic=304652)(phoenixuk confirms : http://forums.xbox-s...ic=310137&st=30)(Flagged GT experience : http://forums.xbox-s...dpost&p=2224563)(5 xboxes, 10 eeproms & 7 hard-drives confirmation : http://forums.xbox-s...dpost&p=2250003)(woodster gets banned w/ flagged GT : http://forums.xbox-s...dpost&p=2259161)(flagged gamertags started with Marriage Theory : http://forums.xbox-s...dpost&p=2382143)Further still, the assumption is that MS has been marrying HDs since one of the Live updates, probably in July 2004. If you upgraded your drive before MS started gathering the
Marriage details then you should also be fine. MS started banning based on the
Marriage data circa November 9th 2004.
(Live 2.0 Rollout Dates + HD scanning article http://forums.xbox-s...dpost&p=1988809 )(Live 3.0 Articles at IGN April 21st http://xbox.ign.com/...8/508307p1.html)(MS on the new Live scans : http://www.forbes.co.../ap1656290.html)So, based on what I can see
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 timeThe rumours that harddrives are being scanned for Halo2 save games, or homebrew software are *completely* unsubstantiated, although it does appear that font-exploits and other soft-mods as well as cheats for XBL games (PGR2) are being scanned for, and are a banning offense. This will not be an issue if you use a modchip and don't cheat

If you do get your xbox + big disk banned you will need to get a new eeprom, hd and gamertag (you only need a new gamertag if you want to use a big disk).
(instructions for unbanning and recovering an xbox : http://forums.xbox-s...howtopic=299557)If you really want, you can just give up and use a stock xbox for XBL, and you will be fine for ever more

...
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, also, people's hard drives are NOT being scanned for their contents!
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, since Xbox Live 3.0 or so was unleashed on the world.
If you're planning to upgrade an Xbox's HD, you need to do it before you first connect it to XBL.
And finally, remember to use an XBL blocking BIOS so you don't get banned for having an active modchip! And never run the MS Dash from your modded bios!...
Confirmations:There have been many confirmations of
Marriage Theory, I plan to link to some of the more clear and detailed posts. If you have a clear confirmation which you feel should be linked, please PM me.
3 extensive tests :
http://forums.xbox-s...howtopic=305792banned then got new eeprom+gt+big disk :
http://forums.xbox-s...howtopic=305867triple confirmation :
http://forums.xbox-s...dpost&p=1998928Survey results lean towards
marriage theory :
http://forums.xbox-s...howtopic=306745banned then got new eeprom+gt+big disk :
http://forums.xbox-s...dpost&p=20021705 confirmations (2 banned,3 not) :
http://forums.xbox-s...dpost&p=1995549 detailed confirmation :
http://forums.xbox-s...howtopic=307503confirmation, drives are not crosslinked to multiple eeproms :
http://forums.xbox-s...howtopic=308409MS insider confirms :
http://forums.xbox-s...howtopic=308428mmacmu1 survey confirms
Marriage Theory :
http://forums.xbox-s...howtopic=308083IntraDream survey confirms
Marriage Theory :
http://intradream.co...wtopic.php?t=20confirmation thread :
http://forums.xbox-s...howtopic=308001double confirmation + support transcript :
http://forums.xbox-s...howtopic=3080049 confirmations! (3 bans, 6 unbans)
http://forums.xbox-s...howtopic=309595Team Xecuter supports
Marriage Theory :
http://www.xbox-scen...pEFqmSCCwfD.phpthorough confirmations :
http://forums.xbox-s...howtopic=331793...
Drive Cross-ReferencingIt appears you can use an unbanned but not virgin HD with a virgin EEPROM but you can't use a non-virgin EEPROM with a different HD (without copying the drive 'disk.bin' info from one to the other)
(Drives aren't being cross-refrenced :
http://forums.xbox-s...dpost&p=2049500)(6 weeks and still good : http://forums.xbox-s...dpost&p=2176441)(large HDs work too : http://forums.xbox-s...dpost&p=2193001)...
local marriage theory seems to be a crock.The Local Marriage Theory
A new theory which is a refinement to the original Marriage Theory has been floated recently, and successful test results are just beginning to come in.
UPDATE: there have been no successful confirmations in a long time, i think the theory might be a crock.
The theory is that MS marries the EEPROM to the HD by writing metadata onto the HD in a part which doesn't get erased by normal formatting (the partition table). If at a later date you connect to live and this data is not there when it should be, or is there when it shouldn't be, or is wrong, then you get banned.
This is an interesting theory because :
1) it explains the results the original Marriage Theory predicts, and
2) it also explains some results that the original Marriage Theory doesn't predict! and
3) it predicts that you can possibly get back on live without acquiring an XBL Virgin HD!
You should be able to virginize your current HD and thus get back on live by rewriting the partition table with a partition table from a virgin stock drive (or just low-level formatting the drive with a program like autoclave).
You can write the partition table using Slayer's EvoX installer's "Prepare New HD" (3b2) option which runs the evox command 'configsector "disk.bin"' to load a partition table from a virgin stock drive (stored in the disk.bin file)
So, the (not 100% confirmed) steps to getting back onto live are
1) get new eeprom
2) re-virginize HD using Slayer's or autoclave (etc)
3) get a new gamertag if your HD is not stock (ie originally shipped inside an xbox by MS)
(LinoX initial post : http://forums.xbox-s...dpost&p=2024516)
(confirmation: http://forums.xbox-s...dpost&p=2035794)
(revirginizing : http://forums.xbox-s...dpost&p=2071375)
(slyt fix confirmation : http://forums.xbox-s...dpost&p=2074361)
(possible refutation : http://forums.xbox-s...howtopic=339221)
(fire and slayer auto-install problems : http://forums.xbox-s...howtopic=338957)
(disk.bin format confirmation : http://forums.xbox-s...howtopic=341832)
(how to revirginize : http://forums.xbox-s...dpost&p=2230602)
(another refutation : http://forums.xbox-s...dpost&p=2239796)...
Xboxdash.xbe Must Be The MS Dash:There is a lot of evidence to suggest that the c:\xboxdash.xbe file *must* be the official MS Dashboard or you will probably get banned. Many people mis-install their hacked dash by renaming their xboxdash to something else, say msxboxdash.xbe, then installing their hacked dash as "xboxdash.xbe". DO NOT DO THIS! You will probably get banned.
A better approach is to use a bios which can load hacked dashes from alternate locations, such as c:\avalaunch.xbe or c:\evoxdash.xbe.
...
Mundane Bans:It seems that with all the attention the new banning systems are receiving newbies are forgetting (or not realizing) about the more mundane methods used for detecting modded xboxs!
You can not go on live with a hacked BIOS as it will be detected and you will be banned. This means you can not play backed up games on live! So you must load your retail kernel to go on live, and to load your retail kernel, your xbox's HD must be locked.
Also, some live games will connect to live automatically so you must use an XBL Blocking BIOS to prevent this, other wise you can get banned without realising it. BUT an XBL Blocking BIOS normally does not prevent the MS dash from connecting to Live, so you MUST NOT use the MS Dash when booted off your hacked BIOS. Nor must you update the dashboard, which will also connect to live, and some modern games will do this without asking. You can even get an XBL blocking BIOS for 1.6 xboxes, so you really have no excuse not to use one.
It is also suggested to set a passcode on your XBL account, which will help prevent accidentally logging in to Live.
If MS sends you an email informing you that you've had your XBL account cancelled that is not actually the same as a ban, its actually worse, and normally happens for swearing, making insulting remarks, cheating, or playing pirated Halo2 on Live.
(Bungie on cheating/abuse related bans : http://bungie.net/Ne...eeklywhatsjan14)...
Tell Me About EEPROMs?:The EEPROM is a small chip on your Xbox motherboard which holds 256bytes of important information, including your Serial Number, MAC address, HD Unlock Key, Video Mode, Etc. When someone says "get a new EEPROM" they actually mean getting a copy of someone elses' EEPROM data, and replacing the data on your EEPROM with that data. If you get a new motherboard, you have a new physical EEPROM.
You can buy EEPROMs for 5-10USD from sellers in the Buy/Sell forum
(xbox-scene buy/sell forum : http://forums.xbox-s...hp?showforum=12)(zodiiak's unbanned EEPROMs : http://forums.xbox-s...howtopic=143336)BTW, as a modder, you should always backup your EEPROM data.
...
Alternative Theories:There are a number of alternative theories floating around, IMHO
Marriage Theory is the most plausible
theory which has the most correct predictions and has stood up the best, I reckon there are probably some other causes of bans, which then lead to a flagged GamerTag and thus extensive scanning, but on the whole,
Marriage Theory is the major thing to be aware of.
(summary of the current theories floating around : http://forums.xbox-s...dpost&p=1994396)...
Account Recovery Speculation:There is a
theory that you can prevent a
Marriage Theory related ban which is going to happen, but which hasn't happened yet, by performing an account recovery before actually getting banned. As far as we know, this is not true.
(account recovery refutation : http://forums.xbox-s...dpost&p=2002938)...
If you want to link to this post use this URL : http://forums.xbox-s...howtopic=304652 