Just out of interest, what's the use-case for this? I can't say I've ever had a problem selecting disks to eject. For those who do, obviously this is a useful hint, but I'm not sure where I could put it to use?

I can't speak for the original poster, but since OS X's default behavior has forsaken disk icons on the Desktop, and relegated mounted Volumes to the (often-hidden) bottom tier of the Finder-window Sidebar, I've had some minor issues managing mounted volumes.

More than once I've unintentionally left disk images mounted for days after installing applications, and I've found it a bit of a nuisance to open a Finder window to unmount external drives. None of this is major hardship or productivity killer (and yes, I know I can restore the icons to the Desktop if I choose), but the ability to check for and remove mounted volumes with a single keyboard shortcut is a welcome convenience.

Cheers,

Jim Akin

Or if you have a laptop with a full desktop that you don't have time to sort, and you just want out of there! (As a matter of fact, the script in my comment might work better for you in this case.)