What format did you put on the drive? (IE:FAT32)? all drives must be FAT32, and another thing, im not sure the USB port has enough power to run a hardrive, i'd try using an inverter that plugs into the lighter or aux power outlet..... unless of course your enclosure for the drive doesnt take external power. and im also not sure what usb standard the headunit supports (IE:usb 2.0, or 1.0), as the info doesnt state that.
i'd like to see it work for you, as im wanting to add a 320 gb HDD to my W72gps as well, and i want to add a 4 port USB hub to mine as well. Ill keep you updated to my progress on my project...... i just won a 5 port usb hub and 320gb HDD on fleabay today, so itll be a week/10 days till i receive them.
good luck, and keep us posted!