Mary's answer is exactly correct.
Wrapping one's head around "Masters" and "Versions" and "Previews" -- all of which show in Aperture as "Images" -- is probably the most difficult task users new to the program face. Unfortunately, while Apple's User Manual (available on-line
) is unusually thoughtful, well-organized, and consistent, it pretty much leaves the user alone to figure it out -- presumable with the plan that most users won't bother.
My suggestion -- fwiw, I am a Certified Apple Aperture Pro, and one of the top posters in Apple's Aperture Support Forum (slim stuff, but not weightless) -- is to jam your tiller hard to one side and steer out of the sea of _file management
_ and into the vast ocean of _image management
_. This is a hard -- and for some, discomfiting -- change of direction to make. Aperture (along with Lightroom) is _an image-manager
_. Once you commit a source file to management by you using Aperture (this is done by importing into the Library), you should use Aperture and Aperture alone to manage it. Importing an image-format file into Aperture creates, in Aperture, a "Master" -- this is a file -- and a "Version" -- also a file, but curiously, a small text file -- and an Image. These are what you manage in Aperture. The Image is the core record in the database (the Library is a database) -- the anchor to which all the information you have about an Image is attached.
Aperture is, of course, non-destructive. Some fear "committing" their life's work to a proprietary program (understandably). The commitment isn't eternal -- it's for as long as you use Aperture. At any time you can reclaim any and all files you have imported, intact, and at any time you can create image-format files (TIFF, JPG) to your own specifications from any adjusted Images you have made using Aperture's photo development tools.
The reason I mention this here is that Ben (Hi) seems to have Aperture set up to manage a sub-set of files he keeps in one data store. This is not a good strategy -- there is no easy way to tell which files are under management by Aperture, and which are not, and one ends up with the work of file-management outside of and in addition to image-management in Aperture. Better to separate one's digital camera files into separate sets, commit to using Aperture on one set, and not use any other programs on the files (and Images) in that set. Even simpler, is to commit the whole set to image-management by Aperture, hoist a colorful spinnaker, and sail forward.