Methods and apparatus are provided for controlling writing and reading of data in an array of A storage fields of a probe-based data storage device in which data is written to and read from the array of storage fields by a corresponding array of probes. One method uses the concept of sub-arrays to provide variable-rate read/write operation. Input data blocks are received for writing to the A-field array, each input data block being writable in A/k.sub.0 storage fields where k.sub.0 is an integer .gtoreq.2. For successive groups of k.sub.0 blocks, the k.sub.0 blocks are written to respective sub-arrays, each of A/k.sub.0 storage fields, of the storage field array by selectively writing at one of a series of rates, ranging from 1 block at a time to k.sub.0 blocks at a time, in dependence on a desired data write-rate. The blocks can also be read from the sub-arrays by selectively reading at one of a series of rates, ranging from 1 sub-array at a time to k.sub.0 sub-arrays at a time, in dependence on a desired data read-rate.