Can't eject disk image from Finder in Mountain Lion

I just got a new MacBook pro. I'm running Mountain Lion (10.8.2). Any time I try to unmount an external hard drive or eject a disk image from the Finder, nothing happens (either clicking the contextual eject icon or right-clicking). The items in question are not in use when I try to eject them. I'm only able to do so with Disk Utility. Any ideas? I've repaired disk permissions but the problem persists.