From Halopedia, the Halo wiki
Memories is an achievement in Halo: The Master Chief Collection. It is awarded for activating the eighth Halo: Combat Evolved Anniversary Terminal on Two Betrayals. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.
The eighth terminal is in the control room at the level's very beginning. Walk around the circular platform and the terminal can be found directly across from the central console.