How do I do this?

The owner of that particular Mac will have created or downloaded a new icon and switched it for his HD – it's simple enough to do...

1. Select the icon you want to change your HD (or folder, app or file) to.
2. Choose "Get Info" from the File Menu, or click command-I.
3. Click the icon in the upper left of the info window.
4. Choose "Copy" from the Edit menu (or type command-C).
5. Select your HD on the desktop and "Get Info" for it.
6. Click the icon in the upper left of the info window.
7. Select "Paste" (command-V) from the Edit Menu.

And there you have it – a new icon for your hard drive.

 

beat me to it, you can always use Candybar if you are going to change a lot of icons or customize your system, it's awesome.

 

Google. It's not difficult.

 

you can find it here