Data nodes were portable storage and processing devices utilized by the Borg to archive and transfer information.
In 2375, the 29th century Borg drone known as One was given Borg data nodes by Seven of Nine from which to assimilate information as she deemed that a direct neural interface was too dangerous. (VOY: "Drone")
Later in 2375, the crew of the USS Voyager managed to salvage several data nodes from the remains of a destroyed Borg probe, aiding in the eventual theft of a transwarp coil, in Operation Fort Knox. (VOY: "Dark Frontier")