1. A portable storage medium, based on USB standard, connected to a USB port of a host computer applying UFI protocol, comprising: a USB connector through which the storage medium is physically connected to the USB port of the host computer; at least one nonvolatile flash memory for storing data transmitted from the host computer; a program storage for storing a predetermined operation program based on USB and UFI, and a controller for controlling the entire operation of the storage medium based on the operation program stored in the program storage, wherein the operation program of the program storage includes a memory processing block for checking and updating header block information of the flash memory, a device recognition processing block for transmitting device descriptor information and interface descriptor information for loading a UFI driver to the host computer, and a UFI processing block for generating predetermined UFI response packets by UFI commands sent from the host computer to transmit them to the host computer.