If you were in the beta that sent out the disc of Reach awhile back, check your email.
The disc can't get title updates so Microsoft sent out Games on Demand codes for another copy - Just got mine
"Earlier this year, you participated in the Xbox LIVE Public Preview program and were provided a copy of Halo: Reach as part of your participation. Unfortunately, this Halo: Reach “preview disc” cannot be updated with a conventional Title Update, causing the game to be unplayable after the recent Halo: Reach Title Update on September 21, 2011. We apologize for any inconvenience this may have caused.
To get you back online with Halo: Reach, below is a code for the Games on Demand version of the game. Once the code is redeemed and your copy of Halo: Reach is downloaded, the game will work as usual, including the features added by the title update. All of your Halo Reach game saves will work normally as well. You may then discard the disc from the Public Preview program"
The disc can't get title updates so Microsoft sent out Games on Demand codes for another copy - Just got mine
"Earlier this year, you participated in the Xbox LIVE Public Preview program and were provided a copy of Halo: Reach as part of your participation. Unfortunately, this Halo: Reach “preview disc” cannot be updated with a conventional Title Update, causing the game to be unplayable after the recent Halo: Reach Title Update on September 21, 2011. We apologize for any inconvenience this may have caused.
To get you back online with Halo: Reach, below is a code for the Games on Demand version of the game. Once the code is redeemed and your copy of Halo: Reach is downloaded, the game will work as usual, including the features added by the title update. All of your Halo Reach game saves will work normally as well. You may then discard the disc from the Public Preview program"