1. A detachable memory module for connecting between a host system and a disk drive system, comprising: a first input/output connector for connecting to the host system, the first input/output connector conforming to a first bus standard; a second input/output connector for connecting to the disk drive system, the second input/output connector conforming to a second bus standard; a nonvolatile memory array contained within a removable memory card; a controller connected to the nonvolatile memory array and interposed between the first input/output connector and the second input/output connector to intercept communication between the first and second input/output connectors, the controller converting signals between the first bus standard and the second bus standard, the controller determining which host data to store in the disk drive and which host data to store in the nonvolatile memory array based on power requirement or a combination of speed and power requirements, the controller receiving commands from the host system including at least one of (a) a command that concerns a key that is stored in the nonvolatile memory array but is not stored in the disk drive system, the key being associated with encrypted data that is stored in the primary bulk storage memory system (b) a command that identifies data according to a unique file identifier that corresponds to logical addresses that are not specific to a file (c) a command that identifies a cache policy to be implemented by the nonvolatile memory array; and a housing that encloses the nonvolatile memory array and the controller.