Server system and control method for storage unit

A server system, comprising:

• a plurality of modular devices, consisting of a plurality of storage units, a plurality of computing units or a combination thereof;
• and
• a connection device, connecting a former modular device and a latter modular device in the modular devices,
• wherein the storage unit in the modular devices comprises a transmission interface expander,
• the transmission interface expander detects whether connects the former modular device, and sets the storage unit as a slave device of the former modular device when the transmission interface expander connects the former modular device, and
• when the transmission interface expander does not connect the fanner modular device, the transmission interface expander is set as a storage node and communicates with an external server, and the storage unit connected behind the transmission interface expander becomes a slave device of the transmission interface expander.