File firstStorage= Environment.getExternalStorageDirectory();
if(firstStorage==null)
    {
     Toast.makeText(this,"Ничего не найдено",Toast.LENGTH_SHORT).show();
     return result;
     }
else
     result.add(firstStorage.getName() );
String rootStr= firstStorage.getParent();
rootFile=null;
if(rootStr!=null && (rootFile= new File(rootStr) ).exists() && !rootFile.isFile() )
    {
     File rootFiles[]= rootFile.listFiles();
     for(File file : rootFiles)
         ...
     }