The support site states - Download and store large files on the 16 GB shared storage space.
The Overdrive has a microSD card slot that you can use for file storage. You can access the storage by mapping a network drive on your computer. Unlike an actual card slot that is built into the computer, the computer would require the mapping of the drive since it is connecting to the Overdrive through the WiFi network it creates. The exact steps are listed in the users manual forthe Overdrive step by step.