Meta boxes not showing on custom post type. Blank page?

Question

I am working on my first plugin. I've got a custom post type, with a meta box which has 4 fields which uses update_post_meta, & these work 100% fine. I have then tried to create a meta box that has repeatable fields, when the user clicks 'add new field'. I want users to be able to add as many fields as they like.

My Question: Below, is my code to add new fields. This just doesn't display in the custom post type when viewed, but I don't get any errors. Please can you have a look at my code below, & advise?

<?php
// START OF CREATE MORE META BOXES AS NEEDED INCLUDING JQUERY 
add_action( 'add_meta_boxes', 'dynamic_add_custom_box' );

/* Do something with the data entered */
add_action( 'save_post', 'dynamic_save_postmeta' );

/* Adds a box to the main column on the post and page edit screens */

function dynamic_add_custom_box() {
    add_meta_box(
            'dynamic_sectionid', 'My Tracks', 'mp_myplugin_textdomain', 'mp_dynamic_inner_custom_box', 'post' );
}

/* Prints the box content  */

function mp_dynamic_inner_custom_box() {
    global $post;
    ?> 
    <div id="meta_inner"> 
        <?php
        // get the saved meta as an array 
        $mpformfield = get_post_meta( $post->ID, 'mpformfield', true );
        $c = 0;
        if ( count( $mpformfield ) > 0 ) {
            foreach ( $mpformfield as $mpfield ) {
                if ( isset( $mpfield['name'] ) ) {
                    printf( '<p> Field Name <input type="text" name="fieldname[%1$s][name]" value="%2$s" /> <br /> </p>' );
                }
            }
        }
        ?>
        <span id="here"></span> 
        <span class="add"><?php 'Add form field'; ?> </span> 
        <script>  
            var $ = jQuery.noConflict(); 
            $(document).ready(function() { 
                var count =  <?php echo $c; ?>; 
                $(".add").click(function() { 
                    count = count + 1; 

                    $('#here').append('<p> Form field <input type="text" name="fieldname['+count+'][name]"  value="" /><span class="remove"> Remove form field </span>  </p>'); 
                    return false; 
                }); 
                $(".remove").live('click', function() { 
                    $(this).parent().remove(); 
                }); 
            }); 
        </script>
    </div> <?php
}

/* when the post is saved, saves our custom data */

function dynamic_save_postmeta( $post_id ) {
    // verify if this is an auto save routine 
    // if it is our form has not been submitted, so we don't want to do anything 
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;

    // Save the data 
    $mpformfield = $_POST['mpformfield'];
    update_post_meta( $post_id, 'mpformfield', $mpformfield );
}

Answer 1

To add a meta box to multiple post types, one add_meta_box per post type is needed. In this example, instead of duplicating the same code, a foreach is used. More post types can be added to the array and the meta box will be added in all of them:

add_action( 'add_meta_boxes', 'add_custom_box_wpse_94701' );

function add_custom_box_wpse_94701() 
{
    // Post types to insert the meta box. Adjust array <-------
    foreach( array( 'post', 'portfolio' ) as $pt )
        add_meta_box(
            'sectionid_wpse_94701',
            __( 'Custom parent' ), 
            'blogroll_box_wpse_94701',
            $pt,
            'side'
        );
}

The example above is taken from this Q&A: List of Posts in a Custom Field

Note that the action hook save_post takes 2 parameters, $post_id and $post_object. The latter contains all the post data (title, content, post type, etc).

There are other checks that can be done inside save_post. Check this search query for examples of adding meta boxes and saving the post data.