Secure digital (SD) is removable memory used by some navigation systems for storing map data. It's about the size of a postage stamp and is widely available from stores that sell consumer electronics.
128MB and 256MB sizes are well suited to storing maps for regional areas. 2GB versions are becoming less expensive, so it may also be practical to use SD memory for storing a full set of maps.
Because SD memory doesn't have any moving parts, it may be better than a hard drive in settings where rough motion and vibration are common (such as an off road vehicle or motorcycle).
SD modules are physically about half the size of CF memory modules but otherwise have the same capabilities for vehicle navigation systems.