A network storage server implements a method to discard sensitive data from a Persistent Point-In-Time Image (PPI). The server first efficiently identifies a dataset containing the sensitive data from a plurality of datasets managed by the PPI. Each of the plurality of datasets is read-only and encrypted with a first encryption key. The server then decrypts each of the plurality of datasets, except the dataset containing the sensitive data, with the first encryption key. The decrypted datasets are re-encrypted with a second encryption key, and copied to a storage structure. Afterward, the first encryption key is shredded.