DIablo 3 install space and running diablo 3 plain from a diablo 3 RoS install

Diablo 3 RoS on Xbox 360 is a whole new game, it's not Diablo 3 + (RoS as an add-on) (like on PC where RoS is a true add-on). Check on the xbox live you can't buy just RoS as a DLC to play it with your Diablo 3 game.

So Diablo 3 ant Diablo 3 RoS disks are not compatible.

However Blizzard update Diablo 3 on Xbox 360 to allow the transfer of character to the RoS version of the game. Take a look at this article.

No, you can not.

Diablo 3 and Diablo 3: Reaper of Souls are completely different games, as far as the Xbox 360 is concerned. Diablo 3: Reaper of Souls likely can not actually differentiate between original content and new content, as even the original content has been altered for the expansions release. Here are a few major changes that would be very difficult to naturally interpret by a computer. Keep in mind that it is not good form to debate the likeliness of interpretation between two consoles, and this only acts as examples of potential issues.

• Changes in Difficulty : In Diablo 3, you can set an overall difficulty. Higher levels of difficulty unlock when you beat Diablo on the previous level of difficulty. In Reaper of Souls, there is no "unlocked difficulty", as the game relies on end-game content, rather then just the replay value of higher levels of difficulty.
• The Mystic : Reaper of Sous adds The Mystic, who can add cosmetic changes to items. As Diablo 3 has no implementation of this feature, it would not be able to interpret these cosmetic changes when playing online with other players.
• New Gear : There is a bunch of near gear in Reaper of Souls, as well as different methods for determining what gear drops, and how gear changes a character. These changes effect the entire game, not just the last act.

I think there has been confusion between you and Microsoft technical support, in the description of the problem. In most situations, the expansion release is simply the original game, with the expansion provided as completely separate content. In this case, there should be no problem. Your disc validates the original game, and allows you to play the original game. The best way to understand this is that if you were able to upgrade to Reaper of Souls with a DLC or disc that required the original Diablo 3 to work, you might have luck.

To quote my own interaction with Microsoft technical support, where I spoke to an agent who has experience with Diablo 3,

Jason C: at 10:01:53 So, that is correct. You cannot use the disc for diablo 3 to access ROS. They are standalone games and that will not let you access the content. If that were the case, they would not have offered the ability to move your characters over to the ROS version. They would have just let it start from where you left off.

Marc_Alx has raised a valid point, he simply provided an incorrect source. You can import your save across to Reaper of Souls, regardless of console generation. In other words, your Diablo 3 characters can be imported into Reaper of Souls on both Xbox 360 and Xbox One.

This forum post details the process. While the primary uer appears to have encountered a bug, I can personally vouch for the process. You simply select "Export Save" from the character select screen, in Diablo 3, and from there you should be able to access the character in Reaper of Souls. A forum moderator actually suggests that save transfer is not required on the same console. Having ran into a slightly similar issue, the best resolution to your primary problem will likely be upgrading to Reaper of Souls, and transfering your character across to the new game.