| 摘要 |
First and second counters are arranged in each file management section. The first counter is incremented in total saving or incremental saving. The second counter is set to be the same value as that of the first counter in total saving. A third counter is arranged in each block management portion. The third counter is set to be the same value as that of the first counter of the file management section when a corresponding block is updated. In incremental saving, only a block in which the value of the third counter is coincident with that of the first counter is saved in a second external storage medium. In since saving, only a block in which the value of the third counter is equal to or larger than that of the second counter is saved in the second external storage medium.
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