I'm running Mac OS 10.8.2.
One of my external image storage drives is not being recognized by LR 3.6. The drive shows up on my desktop and can easily be accessed by other apps. But it's not included in the list of drives in my Folders. All other external drives show up fine.
Interestingly, if I boot up with my back-up external boot drive, it shows up! The back-up and the regular boot-up are identical in every way.