An optical storage device includes a pickup head, an input interface, a memory and a memory control unit. The pickup head is for reading a piece of data from a disk. The input interface has an input data processor, and the input data processor has a buffer for registering the piece of data. The memory has a number of blocks. The memory control unit includes a memory access device and a buffer valid access device. The memory access device is for reading the piece of data from the input data register and writing the piece of data into one of the blocks. The buffer valid check device is for checking whether a next written-in block is a valid block, When the next written-in block is the valid block, the buffer valid check device controls the memory access device to stop writing the piece of data into the next written-in block.