I've spent a few weekends scouring forums and google as I also have a "netmd" minidisc player. At the moment there isn't a method for up/downloading via usb. Just like jebba said, the fileformat is closed. There are a few tools around to help you sniff the packets
whilst the device is using the usb cable. I think it's called usbsniffer. You may want to read the docs and forums at http://www.minidisc.org/
as I have.
They even have a link to the minidisc patents. Very complicated stuff but the patents explain exactly how they work ;). I think that the md may be superceeded by ipods and other portable hdd players. Shame though as I do like the device...