Q: When I switch libraries I lose the majority of the material in my main library and can only restore it by repairing the database. Can anyone tell me why this is happening and how I can resolve the problem? Thanks

Can you explain what you mean by "loose"?

I assume, you have more than on Aperture library, you switch from your main library to a secondary library to view it, then switch back to your main Aperture library.

When you open the main library again, you cannot see most of its contents? Is that what you are saying? And you have to repair the library, to reveal the contents again?

What is your Aperture version btw.?

Have you tried rebuilding the library instead of repairing? And repaired the permissions?

Also try the other measures in the Aperture 3: Troubleshooting Basics: http://support.apple.com/kb/HT3805

Does that sound OK to you?

Try it.

If that does not help, describe exactly, how the images are missing. Do you see empty projects and albums, or are library items missing completely?

I seem to have found a way around it. Rather than switching libraries by going into files/switch libraries I used Aperture Preferences to change them. I've done it a couple of times and it appears to work OK.

That is an interesting finding. It suggests, that your "Preferences" file "com.apple.Aperture.plist" may have been corrupted and you fixed it this way. Can you now switch between libraries the usual way? If not, I'd try to remove the "preferences" file from the user library to the Desktop and relaunch Aperture.

The Aperture 3: Troubleshooting Basics:http://support.apple.com/kb/HT3805

describe how to remove the user preferences.

Great!