You should be able to do the following:
1 - Connect the drive to the computer.
2 - Open "My Computer", and then drag the icon representing the USB/Firewire drive to the dock (make sure "Lock Dragging" is deselected for dock).
3 - If you want to change the icon image, just right-click the dock icon > Dock Entry Properties > Change Image button.
Note - if you save your theme, the icon will remain in place, but will not open any drives until the next time you connect a drive that is assigned that specific drive letter.