hmm, you should just be able to copy and paste, rather than replacing a system file.
Plug in your drive and let the disc mount on the desktop. Get Info for both the drive and your icon. Click the icon preview for your custom icon, then copy and paste it into the Info pane of the drive. That should take care of things.
If you want to change back, I think you can Get Info again, click the icon preview again and simply hit delete. This way, you're not overwriting any files and you keep everything intact.
This is how I changed the icon for my iPod.
Alternatively, you can use something like Candybar. I think this will apply the custom icon to all USB drives though, not just the one.