정확한 답변은 아니지만 참고삼어 제가 사용하는 코딩을 올려드리니
응용하시면 원하시는 답을 찿으실 수 있을듯...

......이전생략
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);       
        requestWindowFeature(Window.FEATURE_CUSTOM_TITLE);
        setContentView(R.layout.main);          
         
        setMakeDir("onAir/brodlogo") ;  //디렉토리 생성
        setFileCopy("brodlogo"); //파일 복사
        setMedScan();//sdcard scan
......이후 생략


 //인자값대로 sdcard에 폴더 생성
 public void setMakeDir(String makeDir) {

  File sdcard = Environment.getExternalStorageDirectory();  
  
  File DirName = new File(sdcard.getAbsolutePath() + File.separator + makeDir);  

  if(!DirName.exists()){  
   showLog("Create DB directory. ", DirName.getAbsolutePath());  
   DirName.mkdirs();
  }    
  
 }
 
 //그림 파일을 복사 (assets -> sdcard)
 private void setFileCopy(String mdir) {

  AssetManager assetManager = getResources().getAssets();
  
  InputStream is = null;
  
  String files[] = null;
    
  long filesize = 0;
    
  FileOutputStream fo = null;
  
  try {
   files = assetManager.list(mdir);   

   for (int i = 0; i < files.length; i++){
    is = assetManager.open(mdir + "/" + files[i], AssetManager.ACCESS_BUFFER);    
    filesize = is.available(); 
    File outfile = new File("sdcard/onAir/" + mdir + "/" + files[i]);
    
    if(outfile.length() != filesize){     
     byte[] tempdata = new byte[(int) filesize];     
     is.read(tempdata);     
     is.close();     
     outfile.createNewFile(); 
     fo = new FileOutputStream(outfile);     
     fo.write(tempdata);     
     fo.close();     
    }   
   }
  } catch (IOException e) {
   e.printStackTrace();
  }     
  
 }
 
 //sdcard 내용을 Re Scan
 public void setMedScan() {
        IntentFilter intentFilter = new IntentFilter(Intent.ACTION_MEDIA_SCANNER_STARTED);
        intentFilter.addAction(Intent.ACTION_MEDIA_SCANNER_FINISHED);
        intentFilter.addDataScheme("file");
        registerReceiver(mReceiver, intentFilter);

        sendBroadcast(new Intent(Intent.ACTION_MEDIA_MOUNTED, Uri.parse("file://"
                + Environment.getExternalStorageDirectory())));  
       
        unregisterReceiver(mReceiver);
 }
 
 //Re Scan 후 결과  리스너
 private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            if (intent.getAction().equals(Intent.ACTION_MEDIA_SCANNER_STARTED)) {
                //showLog("Media Scanner started scanning ", intent.getData().getPath());    
            }
            else if (intent.getAction().equals(Intent.ACTION_MEDIA_SCANNER_FINISHED)) {
                //showLog("Media Scanner finished scanning " , intent.getData().getPath());
            }
        }
    };

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