External drive mounts but doesn't appear in Finder anymore

I have an external hard drive (Drobo 2nd gen) connected to my Mac via Firewire. It used to appear on my desktop and in my Finder sidebar, but now it won't show up in either place (I do have "Hard disks" and "External disks" turned on in the Finder sidebar options). The drive *does* seem to be mounting fine: the drive and partition are visible in Disk Utility and I can browse to it in Finder if I use the Finder's "Go to Folder..." command and type /Volumes/Drobo/. I can read/write to it fine once I've navigated to it. I have the latest version of Drobo's Dashboard utility installed, and my firmware is up to date. Any ideas? Is this a sign that my external drive is starting to die? **EDIT:** I've resolved this problem with the help of Drobo's tech support. See my own accepted answer below. I've revised this answer to make it less Drobo-specific, since it ended up being a filesystem thing.