A method of adding a disk in a redundant array of independent disks (RAID) system with a protection pool of storage units includes receiving a request to add a new physical disk to a storage cluster and partitioning the new physical disk into a set of physical extents. The method further includes allocating one or more physical extents of the set of physical extents of the new physical disk to at least one array of physical extents, wherein the array of physical extents is distributed across physical disks of the storage cluster.