A memory module is interposed between a host and a disk drive. The memory module includes a solid-state nonvolatile memory used for caching data sent by the host for storage in the disk drive. Caching takes place under the control of a memory controller in the memory module and may be transparent to the host. The disk drive may remain spun-down when data is cached, saving power. The destination for host data may be determined based on desired speed, power consumption and expected need for that data. A host may send specific commands to the memory module to enable additional functions.