An input/output (I/O) and disk expansion subsystem is provided for an electronics rack which provides expanded I/O and/or disk storage capabilities to electronic subsystems of the rack. The expansion subsystem includes a subsystem enclosure having first and second sides in opposing relation, with the first side accessible through a front of the rack and the second accessible through a back of the rack when the enclosure resides within the rack. A plurality of field-replaceable units reside within the subsystem enclosure, and are accessible and removable through the first or second sides of the enclosure without removing the enclosure from the rack. The field-replaceable units include an input/output adapter cage(s), a storage device cage, power and control supplies, fan assemblies, and a midplane connector assembly. The adapter cage, storage device cage, and power and control supply dock to and are electrically interconnected by the midplane connector assembly.