Dell EMC PowerVault MD 34XX/38XX Series Storage Arrays Administrator's Guide

Because the storage array supports hot-swappable physical disks, you can add two physical disks at a time for each disk group while the storage array remains online. Data remains accessible on virtual disk groups, virtual disks, and physical disks throughout the operation. The data and increased unused free space are dynamically redistributed across the disk group. RAID characteristics are also reapplied to the disk group as a whole.