How can I fix this "Dropzone already attached" error?

Defining below code globally will help:

Dropzone.autoDiscover = false;

You should use either

var myDropzone = new Dropzone("div#myDrop", { url: "/file/post"});

or

$("div#myDrop").dropzone({ url: "/file/post" });

not both. Basically what you are doing is calling the same thing twice.

Add Dropzone.autoDiscover = false before $(document).ready like this:

Dropzone.autoDiscover = false;
$(document).ready(function () {

});

<script>
  Dropzone.autoDiscover = false;
  $(document).ready(function() {
    var myDrop= new Dropzone("#myDrop", {
      url: '/admin/media'
    });
  });
</script>

instead of

<script>
  $(document).ready(function() {
    Dropzone.autoDiscover = false;
    var myDrop= new Dropzone("#myDrop", {
      url: '/admin/media'
    });
  });
</script>