A data-downloading technique for a data storage device before soldering the data storage device into a product. The data storage device uses a flash memory to provide first-type blocks (using single level cells) and second-type blocks (using multi-level cells). Before soldering the data storage device onto a printed circuit board, a controller of the data storage device allocates the first-type blocks to store data from a host. When the allocated number of first-type blocks reaches an upper limit, the controller changes to allocate the second-type blocks to store data from the host. When detecting that the controller has changed to allocate the flash memory to provide the second-type blocks to receive data from the host, the controller returns a fail message to the host to indicate unreliable write operations prior to soldering.