The phrase "first disc" refers to the initial or beginning part of something, usually in a series or collection. It implies that there are more parts or discs to follow. Full definition
I beat the entire first disc in one afternoon staying at a family friend's house in South Dakota where no one was near my age. (gameological.com)
The FastPlay - enhanced first disc opens with promos for Walt Disney Studio Access, Cinderella: Diamond Edition Blu - ray, Brave, and The Aristocats: Special Edition Blu - ray. (dvdizzy.com)
The main thing I remember about this game was that the UK Official Sega Magazine gave away the entire first disc for free as the cover gift. (gameological.com)