A shunt activation signal is transmitted by an external control terminal through an external transmission interface to switch a flash memory controller in a shunt mode. The shunt activation signal of the external transmission interface can set up a switch as shunt. When the flash memory controller is defective due to errors or damage, the shunt mode enables the external control terminal to directly process data saving/retrieving to the flash memory chip or testing through the external transmission interface. Thus, the user need not purchase a new flash memory to replace the defective flash memory with the damaged flash memory controller.