The Death Star plans were contained on a data-tape cartridge.
Data-tape, which can also be written as datatape or data tape, refers to a type of physical medium used to store various forms of data. This tape, characterized by its continuous, narrow, and flexible form, was housed on reels within a protective casing. Binary tapes that utilize monomolecular-switching, such as those employed by the Galactic Empire on Scarif, possessed a storage capacity of 512 million exanodes.
Data-tapes served as the storage medium for the secret plans detailing the Galactic Empire's formidable planet-destroying weapon, the Death Star. These tapes were stolen, and subsequently downloaded onto a datacard by members of the Rebel Alliance. The datacard was then entrusted to Princess Leia Organa, who transferred the data from the card into the memory systems of the droid R2-D2. This was done to ensure its delivery to Jedi Master Obi-Wan Kenobi on Tatooine.
In the novelization of the original Star Wars film, which was published in 1976 before the film's release, the Death Star plans were described as being stored on data-tapes. The movie, A New Hope, was known simply as Star Wars at the time. While tapes represented a contemporary method of information storage during that era, they eventually became largely outdated for data storage purposes.