M4 field disk
From Halopedia, the Halo wiki
Helmet Recorders use these small recorder chips to store the visual files created by the recorder's optical interface. They can be plugged into nearly any information terminal that can play optical data.
During The Battle of Sigma Octanus IV, the Spartans were wearing these, and the Master Chief's Recorder chip contained the record of his mission to Côte d'Azur, as well as intelligence on Covenant Engineers. These were very well appreciated by HIGHCOM.
Private Jenkins' helmet was equipped with one, and the Master Chief used it to view the squad's progress as he tried to find Captain Keyes. It was by this chip that he first saw the Flood.
