The startup volume should always appear top right of the screen. However, if it's NOT the startup volume, it will be replaced by the startup volume. If your startup volume doesn't appear at top right, then it is a bug in Mac OS X that I'll recommend you to report to Apple.
You probably want it farther right than the finder allows. I'm guessing 'cause I don't know what you mean by moves to the left. In your view options you can set your desktop to keep icons arranged by name/size/etc or by snapping to a grid. If you turn on the keep arranged by grid, and your HD AND other icons stay left, you might want to mess with your screen resolution.