fmount fails with fsMediaError

i want to use keil middleware for file system and using micro sd with ssp on LPC1768. after finit("M0:") i call fmount("M0:"), but it returns fsMediaError error.

#include "cmsis_os.h"
#include "rl_fs.h"
#include "GPIO_LPC17xx.h"

fsStatus state;

/* * main: initialize and start the system */
int main (void) { GPIO_SetDir(2, 0, 1); GPIO_SetDir(2, 1, 1);

if(finit("M0:") == fsOK) { GPIO_PinWrite(2, 0, 1); if(fmount("M0:") == fsMediaError) { GPIO_PinWrite(2, 1, 1); } }

while(1) { }
}