谢谢楼上的帮忙，我已经实现了想要的功能，谢谢。以下是我实现的代码。主程序：

int main(void)
{
    static FATFS fs;            // Work area (file system object) for logical drive
    static FIL  fdst;      // fsrc,file objects
    static BYTE buffer[4096] = {"123 "};   // file copy buffer
    static FRESULT res;         // FatFs function common result code
    static WORD bw;         // File R/W countbr,
    int k = 5,j = 5;   
    SysCtlClockSet(SYSCTL_SYSDIV_1 | SYSCTL_USE_OSC |
                   SYSCTL_XTAL_8MHZ | SYSCTL_OSC_MAIN);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_UART0);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_SSI0);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);
    while(k--)
    {
      f_mount(0, &fs);
      res = f_open(&fdst, "dstfile.txt", FA_OPEN_ALWAYS | FA_WRITE);
      res = f_lseek(&fdst, fdst.fsize);
      int len = strlen(buffer);
      while(5)
      {
        res = f_write(&fdst, buffer,len , &bw);
        f_sync(&fdst);
      }
              
      f_close(&fdst);
      f_mount(0, NULL);
    }
    while(1);
}