File content as of revision 2:23757aa762a7:

#include "mbed.h"
#include "cmsis_os.h"
#include "SDFileSystem.h"

DigitalOut led1(D7);
DigitalOut led2(PD_2);
Serial uart1(D8, D2);                        /* UART1: Modbus           */
Serial uart2(D1, D0);                        /* UART2: pc(USBTX, USBRX) */
                                             /* (i.e.) pc(D1, D0)       */
SDFileSystem sd(D11, D12, D13, D10, "sd");   /* MOSI, MISO, SCK, CS     */
FILE *fp;

void led2_thread(void const *args){
    while (true) {
        led2 = !led2;
        osDelay(1000);
    }
}

void uart1_thread(const void *args){
    uart1.baud(9600);
    while(true){
        uart1.putc(uart1.getc());
    }
}
void uart2_thread(const void *args){
    uart2.baud(9600);
    while(true){
        uart2.putc(uart2.getc());
    }
}

osThreadDef(uart1_thread, osPriorityNormal, DEFAULT_STACK_SIZE);
osThreadDef(uart2_thread, osPriorityNormal, DEFAULT_STACK_SIZE);
osThreadDef(led2_thread, osPriorityNormal, DEFAULT_STACK_SIZE);

int main() {
    osThreadCreate(osThread(led2_thread), NULL);
    osThreadCreate(osThread(uart1_thread), NULL);

    
    wait(1);
    uart2.printf("\r\n");
    wait(1);
    uart2.printf("Initializing...\r\n");
    
    fp = fopen("/sd/hello.txt", "r");
    if (fp != NULL) {
        fclose(fp);
        remove("/sd/hello.txt");
        uart2.printf("Remove an existing file with the same name\r\n");
    }
    
    fp = fopen("/sd/hello.txt", "w");
    if (fp == NULL) {
        uart2.printf("Unable to write the file\r\n");
    } else {
        fprintf(fp, "mbed SDCard application!");
        fclose(fp);
        uart2.printf("File successfully written!\r\n");
    }
    
    osThreadCreate(osThread(uart2_thread), NULL);
        
    /* spin and spin */    
    while(true);
}