The generic idea is that dragging an item to the trash "un-mounts" it from the system. In plain terms, it removes it from your desktop session. IE: Dragging a floppy disk to the trash ejects it (it also ejects the floppy on bootup when it don't contain system information, or always upon shutdown), and dragging a CD, DVD, or other removable device unlocks the drive, and ejects the media. Intuitively likewise, dragging a volume (shared or local) to the trash removes it from your desktop session. Therefore, you can exclude any drive or partition by draggging it out to the trash. Why would you want to do this? Perhaps you don't want that drive to be indexed, or you don't want a utility to search there. Maybe it is a slow drive. I used to have a "dumb" AppleScript that would clean off a drive of things that weren't labeled. If you didn't want it to find a drive, you would just "eject" it (please don't expect a phsycial drive to drop out of the machine).

People don't find these sorts of over-conceptualizations easy to deal with, but when taken in the right light, and when you think the way the mac was designed to be thought about, it does make sense.