Recently, I ran into a problem where my external music drive would show up under Mac Os X in the disk utility and would mount properly there, but wouldn't be visible on the desktop. My google-fu was weak and I searched unsuccessfully for a solution to the problem. Finally, I found a thread on a forum that described this problem exactly, and was able to fix it
I went to :
chflags nohidden /Volumes/yourvolumenamehere/
It works perfectly!