The &xcenter; "Diskfree" widget in "&warpcenter; style" behaves very similar to the monitors in the &warpcenter;, that is, you can click onto the widget to iterate through the available disks.
However, you can also switch the widget to "single" mode in its properties. To do so, right-click onto the widget and select "Properties". You can create several diskfree widgets this way to show you several drives at once.
There is also a
"Diskfree" widget in "condensed style"
available.