Method and apparatus for caching and retaining non-requested speculative data from a storage array in an effort to accommodate future requests for such data. A cache manager stores requested readback data from the storage array to a cache memory, and selectively transfers speculative non-requested readback data to the cache memory in relation to a time parameter and a locality parameter associated with a data structure of which the requested readback data forms a part. The locality parameter preferably comprises a stream count as an incremented count of consecutive read requests for a contiguous data range of the storage array, and the time parameter preferably indicates a time range over which said read requests have been issued. The speculative readback data are transferred when both said parameters fall within a selected threshold range. The data structure preferably comprises a RAID stripe on a selected storage device of the array.