The authors present the notion of a Type-Safe Disk (TSD). Unlike a traditional disk system, a TSD is aware of the pointer relationships between disk blocks that are imposed by higher layers such as the file system. A TSD utilizes this knowledge in two key ways. First, it enables active enforcement of invariants on data access based on the pointer relationships, resulting in better security and integrity. Second, it enables semantics-aware optimizations within the disk system. Through case studies, they demonstrate the benefits of TSDs and show that a TSD presents a simple yet effective general interface to build the next generation of storage systems.