By Default Lion and Mountain Lion hide your internal HDD, External Drives, CD’s/DVD’s, and connected servers from showing up on your desktop. Here we have a Lion Tutorial on how to enable external Discs and Hard Drives on your desktop. When you turn on this feature it will show your internal Hard Drive and any External Discs you have connected to your computer. This makes it easier to access files stored on an external HDD. OSX Lion 10.7 2012
Follow this link:
How To Show External Hard Drives On Your Desktop In Lion