The present invention is to realize a data storage control method and apparatus for an external storage device using flash memories. The method and apparatus can eliminate the data erasure waiting time, eliminating the calculation of the erasure numbers of flash memories, elongating the life-time of the flash memories. Data is stored sequentially from a first flash memory for rewrite data to a third flash memory for rewrite data. When there is no vacant area in the third flash memory for rewrite data, a CPU instructs a first flash memory for garbage collection among the first and second flash memories for garbage collection to perform the garbage collection of the first flash memory for rewrite data. When a host computer issues a write access request, the write process is performed in the first flash memory for garbage collection with the first priority. When the garbage collection of the first flash memory for rewrite data has been completed, the garbage collection of the second flash memory for rewrite data is performed by the second flash memory for garbage collection. At the time of performing the garbage collection of the second flash memory for rewrite data, the data of the first flash memory for rewrite data is simultaneously erased. The first flash memory for garbage collection is changed into a flash memory for rewrite data by the CPU. Upon completion of the garbage collection of the second flash memory for rewrite data, the data thereof is simultaneously erased and the second flash memory for garbage collection is changed into a flash memory for rewrite data. Then, the garbage collection of the third flash memory for rewrite data is performed in the similar manner.