Android 6.0 Permissions Why when I try to create a folder in the SDcard I create it in the internal storage of the device?

First of all I clarify that I am a novice in programming. What I'm trying to do is create a simple folder in the sdcard, but I create it in the internal storage of the device. To do these tests I am using my own Smartphone which is an LG G4 H815 with android 6.0. The app consists of a button, that when pressed creates a folder with the name "PruebaMermisMM". When I start the app apparently everything works fine, I request permission to access photos and multimedia files on the device, but I create it as I said earlier in the internal storage.

In the manifest I have:

<?xml version="1.0" encoding="utf-8"?>

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


<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

In build.gradle (Module: app) I have:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"
    defaultConfig {
        applicationId "lucasandroid.pruebapermisosmm"
        minSdkVersion 18
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:25.1.0'
    testCompile 'junit:junit:4.12'
}

The java code is:

package lucasandroid.pruebapermisosmm;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Build;
import android.os.Environment;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import java.io.File;

public class MainActivity extends AppCompatActivity {

private static final int CODIGO_SOLICITUD_PERMISO=123;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button btn=(Button)findViewById(R.id.btn);

    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            if(checkearPermiso()){
                //Nuestra app tiene permiso
                crearCarpeta();

            }else{
                //Nuestra app no tiene permiso, entonces debo solicitar el mismo
                solicitarPermiso();
            }
        }
    });
}

private boolean checkearPermiso(){


    //Array de permisos
    String[] permisos={Manifest.permission.WRITE_EXTERNAL_STORAGE};

    for(String perms:permisos){
        int res=checkCallingOrSelfPermission(perms);
        if(!(res== PackageManager.PERMISSION_GRANTED)){
            return false;
        }
    }
    return true;

}

private void solicitarPermiso(){

    String[] permisos={Manifest.permission.WRITE_EXTERNAL_STORAGE};

    if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.M){         //Verificamos si la version de android del dispositivo es mayor
       requestPermissions(permisos,CODIGO_SOLICITUD_PERMISO);  //o igual a MarshMallow
    }

}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {

    boolean autorizado= true;   //Si el permiso fue autorizado

    switch (requestCode){
        case CODIGO_SOLICITUD_PERMISO:
            for(int res:grantResults){

                //si el usuario concedió todos los permisos
                autorizado= autorizado && (res == PackageManager.PERMISSION_GRANTED);
            }
            break;

        default:
            //Si el usuario autorizó los permisos
            autorizado= false;
            break;
    }

    if(autorizado){
        //Si el usuario autorizó todos los permisos podemos ejecutar nuestra tarea
        crearCarpeta();
    }else {
        //Se debe alertar al usuario que los permisos no han sido concedidos
        if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.M){
            if(shouldShowRequestPermissionRationale(Manifest.permission.WRITE_EXTERNAL_STORAGE)){
                Toast.makeText(MainActivity.this,"Los permisos de almacenamiento externo fueron denegados",Toast.LENGTH_SHORT).show();
            }
        }
    }
}

private void crearCarpeta(){

    File file= new File(Environment.getExternalStorageDirectory().getAbsolutePath(),"PruebaPermisosMM"); // Creamos un archivo llamado PruebaPermisosMM


    //Verificamos si el archivo fue creado exitosamente

    if(!file.exists()) {
        boolean ff = file.mkdir();
        if (ff){
            Toast.makeText(MainActivity.this,"La carpeta fue creada exitosamente",Toast.LENGTH_SHORT).show();
        }else{
            Toast.makeText(MainActivity.this,"La carpeta no pudo ser creada",Toast.LENGTH_SHORT).show();
        }
    }else{
        Toast.makeText(MainActivity.this,"La carperta ya existe",Toast.LENGTH_SHORT).show();
    }

}
}

I also clarify that when I use the same permission request code for the use of the camera or audio recorder through Intent, it works well for me. I do not know why I have problems reading or writing in the sd.