An SD card is a solid-state device. This means it has no moving parts in order to function. This is a big improvement over older portable storage devices. Floppy disks, for example, had flimsy discs inside which spun at high speeds and were subject to data loss when placed near magnetic fields. CDs and DVDs are larger and less durable, and their storage capability degrades quickly with lots of rewrites. An SD card's components are part of its circuitry, and its packaging is small and durable compared to these predecessors.