It seems that if you try to just drop music files on the disk (after finding the special option to turn on the disk drive functionality), the iPod won't play them back by default. It may work if you drop them into the sekrit F00-F19 hidden folders in the iPod_Control folder that is also hidden by default... but, eh. It's certainly not a procedure given in TFM (you have to hunt around for the "enable disk" option as it is).