I would buy one of the docking stations recommended by Compaq for this machine. That certainly fits.

http://www.pcworld.com/reviews/article/0,aid,122479,00.asp mentions:
"A port on the left side of the notebook lets you attach the $299 HP xb2000 docking station, which adds powerful speakers and a second hard drive that bumps storage up another 160GB."

I can't find anything on compaq.com. But you might, if you do a more thorough search. Or look in the documentation that came with the machine. Or go to the shop where you bought it.

Hope this helps.