A method for controlling a storage system and the storage system using this method are disclosed. In the storage system, at least two memory units share an I/O bus. The shared I/O bus transfers information for each memory unit to execute an operation. The operation has at least one high priority cycle and at least one low priority cycle. When a low priority cycle is overlapped with a high priority cycle, the low priority cycle is suspended, and the high priority cycle is operated first. After the high priority cycle is finished, the suspended low priority cycle is then resumed. By doing so, the shared I/O bus may be used by one memory unit during a busy cycle for another memory unit, during which the latter memory unit does not use the I/O bus. Therefore, the I/O bus can be more efficiently used.