Several times an external [USB] hard drive has accidentally become disconnected without ejecting it [wheelychair + loose cord == oops], and both times I end up with two instances of my main hard drive in the Finder [but not on my desktop.] Both of them function just fine, but they don’t go away if you force quit Finder … they stick around until you reboot.
Edit: This seems to be related to the backup/restore method I use [post detailing that coming soon...] — I had a disk image with the same name as my primary hard drive mounted, and that seems to be the cause of the problem. Remounting the drive and the disk image generally fixed things, however the problem didn’t show up consistently.