SdFat version: 20160719

Assuming the SD is the only SPI device.
Edit DISABLE_CHIP_SELECT to disable another device.

Assuming the SD chip select pin is: 10
Edit SD_CHIP_SELECT to change the SD chip select pin.

type any character to start

init time: 748 ms

Card type: SDHC

Manufacturer ID: 0X28
OEM ID: BE
Product: 68BHO
Version: 0.8
Serial number: 0XD50B3A66
Manufacturing date: 11/2010

cardSize: 32026.66 MB (MB = 1,000,000 bytes)
flashEraseSize: 128 blocks
eraseSingleBlock: true
OCR: 0XC0FF8000

SD Partition Table
part,boot,type,start,length
1,0X0,0XC,8192,62543872
2,0X0,0X0,0,0
3,0X0,0X0,0,0
4,0X0,0X0,0,0

Volume is FAT32
blocksPerCluster: 64
clusterCount: 976992
freeClusters: 976905
freeSpace: 32011.22 MB (MB = 1,000,000 bytes)
fatStartBlock: 9310
fatCount: 2
blocksPerFat: 7633
rootDirStart: 2
dataStartBlock: 24576

type any character to start

cardBegin failed
SD errorCode: 0XA
SD errorData: 0X5

#include <FreeStack.h>
#include <MinimumSerial.h>
#include <SdFat.h>
#include <SdFatConfig.h>
#include <SdFatUtil.h>
#include <SystemInclude.h>

#include <Wire.h>
#include <Adafruit_BMP085.h>

const uint8_t chipSelect = SS;

SdFat sd;
SdFile file;

Adafruit_BMP085 bmp;
float startingAltitude;

void setup() {

  Serial.begin(9600);

  if (!bmp.begin()) {
  Serial.println("Could not find a valid BMP085 sensor, check wiring!");
  }

  float totalAltitude = 0;
  for(int x = 0; x < 10; x++){
    totalAltitude += bmp.readAltitude(102505.98);
  }
  startingAltitude = totalAltitude / 10;

  Serial.println("*****DEBUG MESSAGES*****");
  // Alt
  Serial.print("Starting relative altitude: ");
  Serial.println(startingAltitude);
  // Temp
  Serial.print("Temperature = ");
  Serial.print(bmp.readTemperature());
  Serial.println(" *C");
  // Pressure
  Serial.print("Pressure at sealevel (calculated) = ");
  Serial.print(bmp.readSealevelPressure());
  Serial.println(" Pa");
  Serial.println("************************");

  // SD init
  char fileName[10] = "data.csv";
  delay(1000);
  while (!Serial) {
    SysCall::yield();
  }

  // Initialize the SD card at SPI_HALF_SPEED to avoid bus errors with
  // breadboards.  use SPI_FULL_SPEED for better performance.
  if (!sd.begin(chipSelect, SPI_HALF_SPEED)) {
    sd.initErrorHalt();
  }

  // Open file
  if (!file.open(fileName, O_CREAT | O_WRITE | O_EXCL)) {
    Serial.println("Error opening file!");
  }



}

void loop() {

    Serial.print("Real altitude = ");
    Serial.print(bmp.readAltitude(102505.98));
    Serial.println(" meters");

    float relAltitude = bmp.readAltitude(102505.98) - startingAltitude;

    Serial.print("Relative altitude = ");
    Serial.print(relAltitude);
    Serial.println(" meters");

    // Write into the SD card

    file.print(relAltitude);
    file.close();

    Serial.println();
    delay(100);
}