The disk drives of an array are organized into clusters of multiples of G+1 drives with G same-size indexed subdivisions, where G is the number of members in a protection group. G+1 groupings are created in each cluster, including G groupings distributed over G drives in single subdivision indices and one grouping distributed diagonally over multiple subdivision indices. A single grouping in at least one cluster is configured as spare capacity. Protection groups are located in the other groupings. Drive IO loading is rebalanced by swapping the location of a selected protection group with the location of the spare grouping. The protection group to be relocated may be selected by using LOF scores to identify a cluster with IO loading outlier drives that are overloaded and then calculating IO loading that would result from swapping locations of each protection group in that cluster with the spare grouping.