The system is composed of the storage apparatuses with dynamic chunk allocation capabilities, the centralized management computer. Some storage apparatuses have the extra HDDs or volumes for providing extra capacity (extra chunks), which are concealed by a secret key. The storage apparatus with the closed segment has the key management program and key management table. The centralized management computer has the storage on demand management program, the pending request table, the priority table and the master key table. The storage apparatus may connect to the other storage apparatuses for sharing the extra capacities in the closed segment. The storage apparatus issues the chunk addition request to the centralized management computer. The centralized management computer provides a key according to the priority table and the master key table. The centralized management computer may provide the key according to the chunk consumption rate reported by the storage apparatus and/or remaining capacity in the other storage apparatuses.