OK, this is not hard to do.
1. Find the file/icon that contains the icon you want to use. Select it, and then click File > Get Info.
2. Click the small icon in the upper-left corner of the info window... it will appear to have a small colored halo (probably blue, but it could vary depending on your appearance settings) around it. Click Edit > Copy. Close the info window.
3. Click the icon for your hard drive, and then click File > Get Info.
4. Click the small icon in the upper-left corner of the info window, and then click Edit > Paste.
That's all you have to do!