A method for moving files between network storages in a virtualized network storage system having multiple network storages and rebalancing the free disk spaces. It periodically judges whether to start rebalancing. Two different conditions are adopted for the determination of start: (a) an imbalance of free disk spaces and the minimum value of the free disk spaces is less than a threshold and (b) an imbalance of free disk spaces and the frequency of accesses to the system is less than a threshold. Rebalancing is performed until the difference in free disk spaces falls below the threshold without exceeding the maximum execution time of rebalancing. If a request to write into a file which is being moved from a client takes place during a file moving step, the movement of the file is aborted and the file is deleted from the destination.