Usually, to change the icon of an application, folder, or drive, you can simply Get Info on the item in question, and cut and paste a different icon in there. But in order to do that, you have to have something that has the icon you want -- that you can cut or copy from.
Recently, I restored from a backup, and lost my custom drive icon. On my backup drive, I still had the ".VolumeIcon.icns" file. I copied that to my desktop (renamed to disk.icns), and I could open the .icns file in Preview, IconBrowser, and IconComposer. I could *see* the icon I wanted, but the icon for the actual .icns file itself was the generic Preview icon. I could not figure out how to get the icon that was *inside* the .icns file to be the icon *displayed* by my hard disk icon.
Eventually, I found a utility called icns2icon that you can use to change the *display* icon of the .icns file to match the icon that is contained within. Then you can just use Get Info and cut and paste the way you normally would.
[robg adds: Another way around this would be to use the "capture region to clipboard" keyboard shortcut -- Shift-Control-Command-4 -- and then drag around the visible icon in Preview. Switch to the Finder, do a Get Info on the target, select the icon area, and hit Command-V to paste the clipboard. Just drag carefully to get just the icon itself.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005051604221464