StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath());
double sdAvailSize = (double)stat.getAvailableBlocks()
                   * (double)stat.getBlockSize();
//One binary gigabyte equals 1,073,741,824 bytes.
double gigaAvailable = sdAvailSize / 1073741824;

File dir = new File(Environment.getExternalStorageDirectory().getParent());
            File[] f = dir.listFiles();
            for(File file:f)
        {
            if(file.getName().equals("sdacard0"))//check name of sdcard from  file manager.
            {
                if(file.isDirectory())
                {
                    f = file.listFiles();
                }
            }
        }

System.getenv(ENV_EXTERNAL_STORAGE)

public static File getExternalStorageDirectory() {
    throwIfUserRequired();
    return sCurrentUser.getExternalDirs()[0];
}

/** {@hide} */
public static File getLegacyExternalStorageDirectory() {
    return new File(System.getenv(ENV_EXTERNAL_STORAGE));
}

/** {@hide} */
public static File getLegacyExternalStorageObbDirectory() {
    return buildPath(getLegacyExternalStorageDirectory(), DIR_ANDROID, DIR_OBB);
}