This ThinkSystem DS Series Dual IOM LFF Expansion Unit attaches to the ThinkSystem DS4200 Storage Array to provide additional storage. Each ThinkSystem DS Series Expansion Unit ships with two SAS I/O expansion modules. Each expansion module provides 12 Gb SAS connectivity to the internal drives, and it has three external 12 Gb SAS x4 ports (Mini-SAS HD SFF-8644 connectors labelled Port A, Port B, and Port C) that are used for connections to the ThinkSystem DS4200 and for daisy chaining the expansion enclosures between each other.