Missing internal drive icon on the dock

The icon for one of my internal drives will not show on, nor can I add it to the dock. It was there yesterday until I inadvertently moved it off. I have tried re adding it using the "add to dock" option from the finder (right clicking on the icon there) as well as actually dragging it onto the dock. Neither works. I have rest the NVPRAM, I have powered off the computer, removed the power coord, and the fired things up again. No success. Anyone have any suggestions of what to try next? It's not critical, but is extremely irritating. Thanks in advance.