A storage array includes a scalable drive cluster and non-scaling drive clusters on which RAID (D+P) protection groups are implemented using partition as protection group members. The scalable drive cluster is scaled by adding one or more new drives and moving protection group members onto the new drives. Reserve capacity is calculated based on scaling of the scalable drive cluster by converting unused partitions to reserve capacity when the cluster is scaled. When W=(D+P) new drives are added to the storage array the reserve capacity is replenished, if any was used, by moving protection groups from the scalable drive cluster to a non-scaling drive cluster created with the W new drives. Maintaining the reserve capacity on the scaling drive cluster improves function of the storage array because unutilized reserve capacity can be relocated during scaling without moving data.