There seems to be a lot of confusion with the Gamer Tag issue, so I am going to try to clear up any questions and doubt.
First of all, when we tell you that you must register a new GT to install a non-stock HDD, listen to us! We are right! If you think we are wrong, then PROVE us wrong!
Now, let me explain what we believe is happening with the GTs.
When your Xbox is banned from Xbox Live, MS is BANNING your EEPROM, BANNING your HDD, and FLAGGING your GT. Keep in mind that MS is NOT BANNING your GT, they are FLAGGING it. There is a huge difference!
Everyone with me? Good.
By flagging your GT, MS knows you have been previously banned, and will begin performing more extensive tests on any Xbox connecting with that GT, to determine if it has been modded. Part of these extensive tests is MS checking to see if you have a stock HDD installed. If you don't, then you're banned.
At this point, you're probably asking why MS isn't doing this for all GTs, instead of just certain GTs. I'll tell you. MS has just developed this banning system, and it is far from fully tested. The last thing MS wants to do is accidentally ban a clean Xbox. So for now, they're just running the extensive tests on Xboxes connecting with the flagged GTs. If this is the case, we can all expect it to change in the near future, which will suck beyond all recognition.
In summary, this means that your flagged GT will work in a clean Xbox, but not in a modded Xbox with a non-stock HDD for example.
Now, before you start posting random thoughts, make sure you understand what I just said. If you don't, start from the beginning and read it again. Repeat until every word is memorized. Then, read it twelve more times and you may post.
Let me remind you that none of this is necessarily 100% correct. It is nothing more than what we, or more specifically, I, believe is happening with the flagged GTs. The purpose of this post is to get people thinking slightly deeper into what MS is doing. So please, think before submitting arbitrary posts.