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2.3 Redundant Drive History Modules Overview History Modules are capable of redundant disk drive configurations. The additional redundant Winchester disk drive(s) is located in a second Winchester Drive Module that is stacked above the primary Winchester Drive Module and is physically similar to the WREN III, 210 MB, 445 MB, 875 MB, or 1.8 GB Dual Drive configuration that is illustrated in Figures 2-1 and 2-5. What is redundancy? The term “redundancy” is used to describe redundant WREN III, 210 MB, 445 MB, 875 MB, or 1.8 GB drives, not redundant electronics modules, which is the case with other LCN modules. Although hard drives are quite reliable, the catastrophic nature by which data is lost during a failure indicates a possible need for data to be backed-up during normal operation. Redundancy drive synchronization In a redundant configuration, software does a successful write to one WREN III, 210 MB, 445 MB, 875 MB. or 1.8 GB drive, then does an identical write to its redundant partner to provide two drives with identical databases. The process is called “data synchronization.” If a hard drive “crash” occurs, software ignores the failed drive and continues to operate with its good partner. The redundant hardware is designed so that a maintenance technician can remove the failed drive without disturbing or removing power from its good partner. After the drive has been replaced, the technician performs a “synchronization” task that copies the data to the new drive and restores redundant operation. Drive maintenancewith redundancy Redundancy is designed into the hardware so that a service technician can remove the Winchester Drive Tray that contains the failed drive without disturbing or removing power from its “good” partner mounted on another tray. After the defective drive has been repaired or replaced, the tray is reinstalled, and the technician performs a “data synchronization” procedure that copies the data stored on the “good” (backup) drive to the repaired or replaced drive. Redundancy operation is then restored. To recover from an unrecoverable data error on a drive, bad sectors can be initialized or assigned to the Defect Data List while system operation continues using the “good” drive