1. An in-line storage device for insertion between a disk drive and a disk drive communication bus that communicatively connects the disk drive to a computer, the disk drive comprising a storage disk, a disk drive controller and a disk drive connector for communicatively connecting the disk drive to the disk drive communication bus, the in-line storage device comprising: a housing having a first connector for communicatively connecting the in-line storage device to the disk drive connector and a second connector for communicatively connecting the in-line storage device to the disk drive communication bus; non-volatile (“NV”) memory mounted within the housing; and a controller within the housing configured to operate the NV memory using commands from an operating system of a computer in communication with the disk drive through the disk drive communication bus, wherein the in-line storage device responds to hybrid disk drive commands from the operating system to emulate a hybrid disk drive having both spinning magnetic media and solid state non-volatile memory and wherein the controller is adapted to translate hybrid disk drive commands into non-hybrid disk drive commands for the disk drive controller.