// 쓰기
FileOutputStream fos
= new FileOutputStream(Environment.getExternalStorageDirectory().toString() + "/" + "파일명.dump");
fos.write(buf); //byte[]
fos.close();
//읽기
File file = new File(Environment.getExternalStorageDirectory().toString() + "/" + "파일명.dump");
FileInputStream fis = new FileInputStream(file);
int length = (int) file.length();
byte[] readBuf = new byte[length];
fis.read(readBuf);
fis.close();