Yamaha sell this product to dealers without a HDD if requested and reccomend and give full instructions to chang HDD. It's as easier to fit one in this than in a PC anyway.
You can fit any IDE HDD but if it is over 137GB it will think the drive is 137GB which is all the processor can understand (Something to do with 32bit apparantly)
Because it's recommended by the manufacturer if you follow their instructions you are under warranty. But i'm sure a proper yamaha dealer would fit a drive for you if you batter your eyelids!!