what is your Aperture version?
In Aperture 3 you can turn your Aperture Library into a referenced library - keep the library itself and previews of all images on the system drive, but relocate the original image files for selected older projects to an external drive. As long as the external is connected, you will notice no difference. If your external drive is offline, you can still browse your older projects, tag them and rate them using the previews, but you will not be able to edit them or export them, until you reconnect the external drive.
To relocate the originals select the images in the browser and use the command "File > Relocate Original"; see this paragraph in the User Manual: Aperture 3 User Manual: Working with Referenced Images
But before you restructure your library, be sure to have a working backup of the Aperture library.
Thanks boss! - Thats very helpful!
I had another question for you because you seem to know what you're talking about.. I have 2 profiles on my macbook. One for me, one for my wife.
The actual Aperture library file is located in the Users\Shared folder. In addition to that, under the get info of the file, the permissions are set to read and write for both users.
Now Aperture works great on my profile (usually the one where the pictures uploaded).
When my wife opens aperture on her profile, the application will open, all the projects, albums, pictures ect are all there. However, they are blank - the thumbnail is empty. Sometimes when I click the picture, it will come up, other times it wont. Some times it even says 'Unsupported file type'.
All these pictures work fine on my profile.
Waseem, a Shared folder is not enough to ensure, that two users can share an Aperture Library, because the Library is a package of nested folders and Aperture will set the permissions inside these folders like for a single user library. You need to move the Aperture library to an external drive, a disk partition, or disk images that has the "Ignore Ownership on this volume" flag set. Then two or more persons can access and modify the library, but still not at the same time.
This support article explains how to do this for an iPhoto Library, but the same holds for an Aperture Library, since they both share a unified library format: