A hybrid storage system combining optical disks and magnetic disks is proposed and evaluated via mathematical models. Unlike most current applications of optical disk technology, which consider static databases or deferred update, this research considers environments with a moderate level of near real-time updates. An example of such an environment is databases for administrative decision support systems (DSS). The proposed hybrid storage system uses a write-once, read-many optical disk device (ODD) for the database and a conventional magnetic disk (MD) for storage of a differential file. Periodically, the differential file is used to "refresh" the ODD file by writing updated records to free space on the ODD. When available free space on the ODD is exhausted, the file is written to new ODD media - - a "reorganization" operation. Models of storage cost are developed to determine the optimum refresh cycle time, t*, and optimum reorganization cycle time, T*. Parameters of the model include data file volatility, file size, device costs, and costs for refresh and reorganization. Numerical results indicate that the hybrid system is attractive for a broad range of database environments.