Data crystal

From Memory Alpha, the free Star Trek reference

Data crystals are small, sophisticated information storage devices able to store a large amount of information. They can be connected to dataports. Prior to Arissa's undercover assignment to the Orion Syndicate, which required her memories to be altered, a copy of her memories were stored inside a data crystal so that her identity could be restored after the assignment was complete. (DS9: "A Simple Investigation")