I had a USB pen drive that stopped mounting on the desktop, but I could access it via Go to Folder: /Volumes/DriveName. Then I was talking to someone else and they said they had the same problem. Looking into it, I discovered that somehow a hidden file was created in /Volumes with the name ._DriveName. The way I fixed the problem was to type (in the Terminal) sudo rm /Volumes/._DriveName, where DriveName is the name of your pen drive (an ls -a will let you see the exact filename if you don't remember it). This may also work for other types of media, but I have only encountered this problem with USB pen drives.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030903105801807