Multiple-disk organizations can be used to improve the I/O performance of problems like external merging. Concurrency can be introduced by overlapping I/O requests at different disks and by prefetching additional blocks on each I/O operation. To support this prefetching, a memory cache is required. Markov models for two prefetching strategies are developed and analyzed. Closed-form expressions for the average parallelism obtainable for a given cache size and number of disks are derived for both prefetching strategies. These analytic results are confirmed by simulation.