foo.ino

#include <SD.h>

const int sdChipSelect = 10;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);

  if (!SD.begin(sdChipSelect)) {
    while(1) {}
  }

  char filename[13];
  findFilename2(filename);
  Serial.print("outside:");
  Serial.println(filename);

  File newOne = SD.open(filename, FILE_WRITE);
  newOne.write("hi");
  newOne.close();

  Serial.println("done");
}


void loop() {
  // put your main code here, to run repeatedly:

}

void findFilename2(char* filename) {
  filename = "FLIGHT00.csv";
  for (uint8_t i = 0; i < 100; i++) {
    if (i < 10) {
      itoa(i, filename + 7 * sizeof(char), 10);
    } else {
      itoa(i, filename + 6 * sizeof(char), 10);
    }
    filename[8] = '.'; //clear null termination from itoa

    if (!(SD.exists(filename))) {
      Serial.print("inside: ");
      Serial.println(filename);

      break;
    }
  }
}

output.txt

inside: FLIGHT04.csv
outside:
done