I'm not sure if you can turn that off or not, synergy. They show up there because they are hot-swappable, like Him said. Although you can't disconnect your primary drive, you can readily pop-off your secondaries and attach new ones at will. Which is cool.
I'd be willing to bet you might turn something up if you searched through Microsoft's knowledge-base, though. There's usually tons of unknown tweaks and stuff to find.