I don't know the answer to that question, but I will try to research it. it is the storage system which powers the U.S. Library of Congress on line system.

Even if the storage system of the U.S. Library of Congress is an off-line system, it needs backup. For electronic data archiving, the golden rule is: Two copies stored in separate locations. By the way, if you have to design an electronic archiving system, avoid to replicate the archiving storage in real time. Otherwise, if the archive repository suffers a ransomware attack, the real time replication mechanism will destroy the second archive repository ... in real time.