It had an unlimited amount of virtual memory, and used no physical storage device. It was supposed to be impossible for an organic being to retrieve deleted data from a Standard Unlimited Virtual memory.
The Starbase 36 computer system used Standard Unlimited Virtual memory. In 2364, Lieutenant Commander Data interfaced directly with the Starbase 36 computer to investigate tampering, where he discovered two conflicting memories at the same location and time. As an android, he was able to recover the deleted file and expose the crime. (TNG novel: Survivors)