You can edit the icons for different drives the same way as for Windows/Explorer.
Or you could use individual Go buttons for each drive, instead of the Go DRIVEBUTTONS command which makes the list of drives, so that you have full control over their icons, labels, separators, etc.
(If you just want separators between types of drives, not between drives of the same type, you can do that by using the drive-type arguments to the Go DRIVEBUTTONS command.)