Under System 6, changing the icon on any icon required a utility like ResEdit to modify. The location of the hard drive icon depended on where the manufacturer decided to put it. For example, with La Cie drives, the icon is made available to users for changing.
For Apple drives there was a custom resource in the SCSI Setup that puts the icon in the boot blocks of the drive. Changing the hard drive icon using a resource editing utility is not recommended by Apple.
With System 7 systems, changing the drive icon is as easy as copying an icon or picture into the Clipboard, opening Get Info on the drive, clicking on the icon, and pasting the icon you have in your Clipboard. You will not be able to change the icon if the drive is shared, or locked.