SetFile can be used, if you have the Developer Tools installed, to hide selected drives. A previous hint from discussed using SetFile to hide the swap partition, but it works on the startup disk as well. In this case, one types:
% SetFile -a V /Volumes/SwapThis makes the swap volume and the root volume invisible, but still shows mounted external hard drives when they are present.
% SetFile -a V /
In dialog boxes, with Finder key commands, and the like, the computer operates normally and all partitions are visible, but I don't have my desk cluttered with drives that I know are there already. And when I have my backup drive installed, it shows up on my desktop just as it ought to.
Interestingly, when you select "Computer" from the "Go" menu on the finder, the only thing that appears is the network globe. Is this ever actually useful for anything?

