A memory device, includes a non-volatile semiconductor memory including a plurality of first areas, each corresponding to an erasing unit, each of the first areas including a plurality of second areas, each corresponding to a writing unit; and a controller circuitry configured to erase data stored in a first area of the non-volatile semiconductor memory, track amount of time elapsed since the erasing data from the first area, write first data into one or more unwritten second areas of the first area before the tracked amount of time reaches a particular time period, and write second data into one or more unwritten second areas of the first area in response to the tracked amount of time reaching the particular time period, each of the writing of the first data and the writing of the second data being carried out independently of an instruction from a host.
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