Exporting DB from android device sd can't write

Hello I have this code

public void onbtnExportDBToPCClick(View view){

    try {
        File sd = Environment.getExternalStorageDirectory();
        File data = Environment.getDataDirectory();

        Toast.makeText(getBaseContext(), data.toString(),
                Toast.LENGTH_LONG).show();
        if (sd.canWrite()) {
            String currentDBPath = "//"+data+"//" + getPackageName() + "//databases//"+"cDataBase.db";
            String backupDBPath = "//backupcDataBase.db";
            File currentDB = new File(currentDBPath);
            File backupDB = new File(sd, backupDBPath);
            Toast.makeText(getBaseContext(), currentDBPath.toString(),
                    Toast.LENGTH_LONG).show();

            if (currentDB.exists()) {
                FileChannel src = new FileInputStream(currentDB).getChannel();
                FileChannel dst = new FileOutputStream(backupDB).getChannel();
                dst.transferFrom(src, 0, src.size());
                src.close();
                dst.close();
                Toast.makeText(getBaseContext(), backupDB.toString(),
                        Toast.LENGTH_LONG).show();

            }
        }
    } catch (Exception e) {

        Toast.makeText(getBaseContext(), e.toString(), Toast.LENGTH_LONG)
                .show();

   }

I have set permitions

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" ></uses-permission>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" > </uses-permission>

but code stops at if (sd.canWrite()) it does not go further. I don't know what is the problem. What else should I have to set?