So you are copying the folders into a folder 2 levels deep, with a split level of 1 on the share? That means that its going to copy them onto the disk that owns the folder you are copying them into, in your case the category folder. Split level of 1 means it will only split the first level of folders under the share, any deeper than that will stay on the same disk. So all the music within each category in your case are on the same disk.
Let me know if I have misinterpreted what you said though.