This paper presents Dynamic Chunk Rotation (DCR), a storage-layer moving target defense mechanism for distributed file systems. DCR periodically migrates data chunks across storage nodes, causing attacker topology knowledge to decay over time and creating a structural gap in the ransomware kill chain at the lateral movement-to-execution phase. We develop a formal knowledge decay model, derive bounds on attacker success probability as a function of rotation parameters, and analyze the security-overhead tradeoff. An adaptive tier-escalation daemon (Lorraine) provides threat-responsive rotation frequency control. Proof-of-concept implementation on a 7-node MooseFS cluster demonstrates empirical validation of the decay curve across dwell times of 0 through 20 rotation cycles. Two U.S. provisional patents pending.