A mapped RAID system that reduces drive extent allocation changes when splitting a group of storage drives into a first partnership group and a second partnership group. A first drive is assigned to the first partnership group, and then additional drives are assigned to the first partnership group from the unassigned drives, such that each drive assigned to the first partnership group has a lowest movement count of all unassigned drives. A drive's movement count represents a count of drive extent allocations that must be changed if the drive is added to the first partnership group. After each drive is assigned to the first partnership group, the indication of the drive is removed from the list of unassigned drives. A group of RAID extent entries for the first partnership group consists of RAID extent entries indicating relatively higher total numbers of drive extents located in the first partnership group.