Finally
I GOT IT!!
..and I can confirm the pseudo-marriage theory!
Actually MS SERVERS DON'T STORE your HD serial!
When you register with live YOU SIMPLY GET TWO "COOKIES" stored in an hidden place of your HD!!!
If you replace your HDD after registering with live your cookie is lost...and you'll be banned!
If you change your eeprom without changing the HDD the cookie mismatch and you will be banned..
This respects the marriage theory!

Why am I so sure?
I followed some steps...On my moddex XBOX..
0) I did the backup of eeprom.bin/hddkey.bin/disk.bin with slayers' evox installer 2.6
1) backed up my C: and E: drives with FTP
2) I deleted EVERYTHING on both C and E (I had no F or G)
3) I restored the DEFAULT files on C/E with a copy I did the first time I installed my xbit
4) At this point I have a modded xbox with the "pseudo"retail HD
5) I disabled xbit & reboot
6) msdash opened
7) inserted halo2 original disc
8) msdash updated to an msdash with live support
9) registered with live
10) played in LIVE for more than 1 week
11) disconnected the network cable
12) reactivated xbit
13) rebooted with slayers' evox installer
14) did a backup and.... WOW!!! in disk.bin I SAW SOME NEW VERY INTERESTING THINGS!
If you open disk.bin you can see something similar to 2 cookies:
the first contains the xbox serial number (look for "SN", or "xbox.com")
the second contains your gamertag (look for it)
and then was the light!
15) Unlocked the HDD
16) Installed it in my PC
17) Did a SECTOR-BY-SECTOR COPY to a 120GB HD
18) installed the 120GB HD in the xbox
19) LOCKED DRIVE
20) Already on live
21) STILL PLAYING! NO BANS!
with the sector-copying..I SIMPLY COPIED THE COOKIES! to the newer HD.
Probably i would get the same result replacing the disk.bin in the slayer's CD with mine and doing a FULL-RESTORE TO A LARGE HD. However I didn't try.
I'm not sure if M$ will do more check in the near future (for example a third cookie with the HDD serial) but actually this works.
Hope this helps, guys.
This post has been edited by LinoX: Nov 22 2004, 01:07 AM