The 3Com SuperStack 3 NBX V5000 Call Processor Disk Mirroring Kit (3C10207) aftermarket upgrade kit will create a "mirrored" copy of the system configuration file, AA/VM prompts and greetings, and all voice messages in near real-time. Should primary disk fail, system will automatically fail over to the second disk, minimizing any service disruption. Kit includes one (1) NBX system hard disk and a license key-code, both are required to activate.