1. A memory unit connectable to a host system comprising; a substrate; an interface unit formed to the substrate; first and second non-volatile memory chips formed to the substrate; and a memory management unit formed to the substrate and connected to the first and second non-volatile memory chips and the interface unit, the memory management unit comprising, an access unit reading data from memory areas of the first and second non-volatile memory chips at reading, and writing data to the memory areas of the first and second non-volatile memory chips at writing, wherein the first non-volatile memory chip is swappable when the second non-volatile memory chip is operating.