If you have a flash drive / iPhone / etc. you can tap the Magic Mouse / Trackpad one time with two fingers and a drop down menu gives an option to eject.
Now that I look at it - don't see that option when my hdd is highlighted
With some drive types 'eject' will render a disk invisible to Disk Utility, while 'unmount' will not. The classic examples of this are floppies, CDs, and DVDs, but I have run across a few hard drives that behave this way.
If you have a flash drive / iPhone / etc. you can tap the Magic Mouse / Trackpad one time with two fingers and a drop down menu gives an option to eject.