SpaceControl also have support to warn the user for other Volumes then the start up disk.
SpaceControl is designed to be simple. Download the application, unpack the file and drag the application (the one with the hard disk icon) to your Applications folder. Launch SpaceControl. A text will appear in your menu bar showing how much free space there are available on your startup disk. Click on the menu item and submenus will apere, showing information about all disks or volumes mounted on your desktop.
If you want SpaceControl to start everytime you log o to your Mac you can choose that option in the Preference window. You can also make some choises about how you want the menu item to appear.