hmm...well you could do this a lot of ways...if you want you could just make an alias of the drive and throw it in the dock. If you want to make it so that all your drives show up in the dock under a folder (like cds and stuff too) you'll want to make a symbolic link of the Volumes folder. instructions are below:
open terminal
cd ~/Desktop
ln -s /Volumes myVolumes
then an alias named myVolumes will appear on the desktop just drop in the dock (then you can delete the alias on your desktop if you want). Note this will not show your startup disk. I'd suggest just making an alias of it and have it as a seperate item in the dock...if you want you could make an alias and throw it in the Volumes folder...but that could be bad