Why Correct: USB drives function as secondary storage—non-volatile, removable media for data storage accessible by CPU but separate from main memory.
Distractor Analysis: Primary storage comprises volatile, directly CPU-accessible memory like RAM and processor cache. Tertiary storage involves automated robotic systems accessing removable media such as tape libraries.
Takeaway: Storage hierarchy progresses from primary (RAM/cache) to secondary (HDD/SSD/USB/optical) to tertiary (tape archives/automated optical systems).