File content as of revision 3:a9c142f24ede:

#include "mbed.h"
#include "SDFileSystem.h"
#include <string>

SDFileSystem sd(PTE3, PTE1, PTE2, PTE4, "sd"); //mosi, miso, sclk, cs
Serial pc(USBTX, USBRX);
char words[4096];
int n,c;
char buffer[2048];
char received[8192];
bool ontvangen;
int main()
{
    while(1) {
        /*//reading
        FILE *fp1 =fopen("/sd/test.json",  "r");
        if(fp1==NULL) {
            error("Could not open file for read!\r\n");
        } else {
            printf("Reading from SD card...\r\n");
            while((c=fgetc(fp1)) && c!=EOF) {
                words[n]=c;
                n++;
            }
            printf(words);
        }*/
        //writing
        while(pc.readable()) {
            pc.gets(buffer, 2);
            if(buffer=="") {
                printf("Niks doorgestuurd");
            } else {
                strcat(received, buffer);
                ontvangen = 1;
            }
        }
        if(ontvangen) {
            ontvangen = 0;
            FILE *fp = fopen("/sd/test.json",  "w");; //w overwrite's, a appands to file
            if(fp == NULL) {
                error("Could not open file for write!\r\n");
            } else {
                printf("Writing to SD card...\r\n");
                fprintf(fp, received);
                fclose(fp);
            }
        }
    }
}