I, too, have noticed that some external drives show up on the desktop, others not. I have two 1TB backup drives, both formatted in exFat. One shows up when mounted, the other doesn't. When unmounted, both show up.
Most of the USB drives show up, just not all. Which is strange.
They are available in the device manager view of Konqueror, and there's always "unmount".
Its the variety that makes life spicy. If everything always did what you expected, where would the fun be?
Curt-