OSD

An OSD - Optical Storage Device - is a small, portable data storage unit, the equivalent of a disk. Commander Shepard often uses them to transfer data between computers.

Notable OSDs

 * Fist's OSD. Found in Fist's office after the crime boss is confronted. It contains incriminating files about corruption on the Citadel, and is used in the assignment Citadel: Reporter's Request.
 * Qui'in's OSD. Obtained from Lorik Qui'in if Shepard decides to help him. This OSD has an encryption key to access restricted data from Qui'in's computer regarding Administrator Anoleis' corruption.
 * Benezia's OSD. Given to Shepard by Matriarch Benezia. It contains the coordinates for the Mu Relay, to be transferred to the Normandy's navigation computer.

Source
Galactic Codex: Essentials Edition 2183