Transfer drives are a portable storage medium used by Transformers. They work in much the same way as the USB drives that humans use, as they enable a user to download data from a Cybertronian computer system.
Ratchet supplied Agent Fowler with a transfer drive for a mission to the Nemesis to try and download the Iacon database. Once they were on board the ship, the kids helped Fowler find a terminal and plug in the drive, which began downloading data into itself. By the time the kids reversed the energon flow in the ship, shutting down the intelligence running it, the drive had still only downloaded a fraction of the database. Jack and Ratchet managed to retrieve the drive and the data it contained, including four sets of coordinates corresponding to the locations of artifacts originally held at Iacon.