Fast SD Card Data logging

Other Hardware Storage
2

Use write.
It can be like this

#include <SPI.h>
#include <SD.h>
File dataFile;
union 
{ char dta[16];
  struct  {
   long my_time;
   int ac_x, ac_y, ac_z, gy_x, gy_y, gy_z;
  };
} my_data;
//------------- then
void setup() {}
void loop() 
{
  my_data.my_time=millis();
  // my_data.ac_x=accel.x(); // lines like theese
  // 5 more of theese
  // open logfile -> I'd use a counter/timer to open/close file epproc every 10 sec
  dataFile = SD.open("datalog.dta"); // now not a textfile
  // print all data to file
  for (byte i=0;i<16;i++) dataFile.write(my_data.dta[i]);
  dataFile.close();
}

or you can write witout storing values first:
dataFile.write(highByte(ac_x=accel.x()));
dataFile.write(lowByte(ac_x=accel.x()));