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Sorry, I was being quite brief. If you select the drive on the desktop by clicking on it once, you can then "Get info" about the drive from the finder "File" menu at the menu bar at the top of the screen. That will open the Info window for your hard drive, from there you will see the same image for your drive in the upper left hand corner of it's "Get info" window, you can pretty much drag and drop another image to that little image window to change the drives icon image.

I hope that is more helpful.
 
I guess I was being hopeful with the drag and drop technique, sorry about that. What will work is to do some cut and pasting between the get info windows of the get info of the image file you are trying to copy for your new hard drive icon image and the get info window of your hard drive, if you click on the image in the get info boxes you will see that it gets "selected" then you can cut and paste the images.
 
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