HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\DriveIcons

1. Under the DriveIcons subkey, create a new subkey and use the drive letter (ex: D ) that you want to change the icon for.
2. create a new subkey and name it DefaultIcon
3. modify the (Default) value. Double click it and set its value to the full path to your custom icon file. In my case, I will use the file named "Longhorn Drive.ico" which I have in the C:\icons folder