Read more to open external link

You just need to add target="_blank" to the anchor tag.

function remove_more_link_scroll( $link ) {
$link = str_replace('>',' target="_blank">', $link);
return $link;
}
add_filter( 'the_content_more_link', 'remove_more_link_scroll' );

I feel compelled to state that I consider forcing new tabs/windows to be very unfriendly behavior, and if you search the web you will see that I am not alone in that opinion.