You can use CopyIcon in the OS3.9 contributions folder (I think that's where it is,) or you can open the Information for a drive and drag in the icon that you want over the one in the Info window.
But, yes, the disk.info for the particular drive will work, though you may have to reboot or get Workbench to refresh or redraw icons (Reset WB might work, too.) I'm playing from memory, so please forgive my unsurity. :-) (It might be a word.)
