The drives that are partitioned for use in the root aggregate depend on the system configuration. Knowing how many drives are used for the root aggregate helps you to determine how much of the drives' capacity is reserved for the root partition, and how much is available for use in a data aggregate.
Root-data partitioning is supported for two types of platform: entry-level platforms, and All-Flash FAS (AFF) platforms.
For entry-level platforms, only the internal drives are partitioned.
For AFF platforms, all drives that are attached to the controller when the system is initialized are partitioned, up to a limit of 24 per node. Drives that are added after system configuration are not partitioned.