As the identity of the AI I'm asking about is a major plot point in the game Halo: Reach and as I've just recently finished it, I've decided to keep the identity of the AI a secret using spoiler code in my question.
In the original Halo Trilogy Cortana is carried in a SD-like chip that fits into the back of the Master Chief's helmet. In Halo: Reach the player is given Cortana in a large canister that they attach to their lower back.
I know the Spartans in Halo: Reach are of the third generation of Spartans that were created and that the Master Chief is a second generation Spartan.
What is the in-universe explanation for the differences in their ability to carry AI? Do the third generation spartans have a different type of armor or were they securing the AI in that manner for another reason?
I've considered that perhaps the Master Chief only uses the chip because he's exiting the ship in an emergency setting and that maybe the canister is the standard way to move expensive starship class AI such as Cortana.
UPDATE - The Halo Wiki says that "[Noble] team was issued with the Mark V MJOLNIR armor on November 24, 2551." This is the same Armor Master Chief is equipped with in Halo: Combat Evolved.
"Reach takes place...during the year 2552".
The Mark V MJOLNIR page says that the armor has a "revolutionary crystalline layer forms a network, capable of supporting the kinds of artificial intelligence usually reserved for warships, on a piggyback system."
"The synthesis of an artificial intelligence with the human brain is not possible without an upgrade to the user's standard Neural Interface. This specialized version of the neural lace is issued to all SPARTAN-IIs before they can interface with the Mark V. Like the standard Neural Interface, the specialized neural lace translates electrochemical signals to digital code and routes them through an interface connection at the rear of the skull. Through this interface, the user's thoughts command the armor's movement and weapons; and input from the on board sensors comes directly to the user's mind. In addition, an AI personality and processing matrices can be carried by the armor and delivered to the suit via the specialized neural lace and on board storage in a crystal data chip no larger than a personal credit card. Without the armor, a SPARTAN's reaction time is charted at twenty milliseconds; with the Mark V, and a AI installed, the time translating thought into motion is rendered almost instantaneous."
It goes on to say "[Master Chief] was the first Spartan to test the MJOLNIR Mark V with a Smart AI implemented within the armor during a "live-fire" combat scenario."
The novel Halo: The Fall of Reach places this "live-fire" combat scenario during 2552 before the events of Halo: Reach.
This suggests that not only does Noble team have the ability to carry warship AI, but that doing so has been tested successfully on the same planet they are operating on and(!) is proven to greatly improve the reaction speed and awareness of the recipient of the AI.
Integrating the AI with the MJOLNIR armor would greatly increase the chances of the Spartan making it to the delivery location in one piece while the AI would be safely stored inside heavy armor protected by an energy shield.