I had this problem back in Lion days periodically but was surprised to see it again today after having upgraded to Mountain Lion when it came out. I have about six volumes that are mounted at boot time with a script. They always mount up fine and they are not ejected (running df in the console as well as drilling into the 'shared' section of the finder both show they are mounted and available). Today, mysteriously the icons in the sidebar for these shares have all vanished. I went into sidebar Finder preferences and confirmed that 'external discs' for the sidebar is checked. Interestingly if I go into the Finder general preferences and choose to display "connected servers" all of the volumes pop like magic onto the desktop. Is there something flaky with the Finder in Mountain Lion (and maybe in Lion also) or is there some value somewhere that has gotten corrupted that could be causing this to periodically happen? I'm sure a reboot would cure it, but if possible I'd like to get it fixed up for good.