external hard drives not showing up in Finder sidebar

It seems that whenever I inset and external hard drive into my mac, it doesn't show up under the devices list in my Finder's sidebar. All other external devices show up there (such as my ipod, iphone and CDs/DVDs). The preferences are set to show external hard drives. Does anyone know why this is, and how it can be fixed? I don't know of any other way to open up these hard drives.