Fault tolerance in a redundant array 13 of disk drives 20, 21, 22, 23, 24 is degraded when errors exist in the array. Data in the array is rebuilt to remove the degradation, either for entire disk drives or for partial data, with minimal impact on array resources. Rebuilding occurs during idle time of the array or is interleaved between current data area accessing operations of the array at a rate which is inversely proportional to the activity level of the array. Alternatively, data is rebuilt when a data area being accessed is in need of rebuilding.