/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/com.apple.macbookair.icns

When you navigate as far as the CoreServices folder, Control-click on CoreType.bundle and select "Show package contents" to access the Contents and Resources folders. Once you're in the Resources folder, you will see a bunch of Apple hardware icons in there. Drag the one that matches your computer to your desktop. It will make a copy of it there.

Now if you highlight the icon and do a "get Info" to copy and paste it to your default generic hard drive icon, you will find that won't work. All you get it a generic icon to paste. The trick is to download and use a demo copy of Candy Bar.

Next, open CandyBar and drag the computer icon you copied to your desktop previously into the Quick Drop box, as pictured below.

Finally, drag your default generic hard drive icon right over the top of your computer icon in the box above. Whola! Here is a picture of the finished product.