There's nothing special about the drive toolbar; it's just a generic toolbar called "Drives". If you go into Customize mode and edit it you'll find one or more buttons on it which run variations of the Go DRIVEBUTTONS command. That command is documented in the Opus Raw Commands part of the manual.
Like abr says, you can add cdrom,removable to the list of drive types. You can also remove the existing drive types (probably just fixed) so that no types are specified, in which case the command will list all drives.