everytime I open Aperture it starts rebuilding previews and thumbnails. Any idea why?

Have you recently imported a large number of images that still need the generation of previews? That will take some time.

You can prevent Aperture from creating previews at launch, by holding down the "Shift" key while you are launching Aperture. That will defer the preview generation, but is only a temporary solution.

Do you want high resolution previews of your images? That is recommended, but will take some time to render. If you do not want previews, disable the generation of previews in the "Previews" preferences pane, also for each project by selecting it in the "Projects" section of the Library Inspector. The "Mantain Previews for Project" option is hidden behind the cogwheel in the upper right corner of the panel.

Sometimes the generation of previews will hang, if you have a corruption of the library or imported an image or video that cannot be processed.

To repair your Aperture Library, see:

Aperture 3 User Manual: Repairing and Rebuilding Your Aperture Library

To check for images, that cannot be processed, see this thread:

Re: Aperture in Processing.... mode on startup

Regards

Léonie

Closing and reopening Aperture after it completed its job just started rebuilding previews & thumbnails again and again

If all previews have been generated, and you still see frequent processing of previews, there may be two reasons:

• Your Preferences settings do not take, because the preferences file in the user library is corrupted.
• You may have imported a corrupted image file or video, for that the generation of previews will never succeed.

The first can be fixed by removing the Aperture preferences apple.com.Aperture.plist from your user library, see the Aperture 3: Troubleshooting Basics on how to do this.

The second requires to search your last import, and to try to generate previews manually for subsets of these images, to find the one, for which the generation of previews will hang.

Regards

Léonie