1. A data storage device, comprising: a FLASH memory, comprising a plurality of blocks each with a plurality of pages, wherein the blocks of the FLASH memory are grouped to be accessed via different channels; a controller coupled to the FLASH memory, comprising: a processing unit; a read only memory, loaded with codes which are executed by the processing unit as firmware of the data storage device; and a random access memory, allocated by the processing unit executing the firmware, to provide a cache space which comprises a plurality of cache areas caching write data for the different channels, respectively, wherein: by the processing unit executing the firmware, data issued from a host is distributed to correspond to the different channels; and when a logical address requested to be written with data is identical to a logical address that a latest-updated cache area of the cache space corresponds to, the processing unit executing the firmware reuses the latest-updated cache area to cache write data.