This is a great isolation trouble-shooting step. If you can’t fix the problem with something as simple as trashing the preferences, it’s a good idea to figure out if the problem is with your Library, or with Aperture, or with something in your user folder, or even with the OS itself.

(You should have already trashed preferences and cache before proceeding)

The basic premise here is to isolate and eliminate.

New Library

IDEA: If you create a new Library and the problem goes away, you know the problem is in your original Library. If the problem is still there, then it’s a deeper issue.

STEPS: Create a new Library from the menu Aperture > Switch to Library > Other/New… then click the Create New button. Name it something like “temp” and even put it on your Desktop, and see if you can replicate the problem. You may need to import some photos to test with. If the problem is gone, you know where it lies. If not, then continue.

To switch back to your own Library, go back to the Switch to Library menu.

New Aperture Support Folder and New Aperture

IDEA: There are all kinds of user generated presets and settings stored in the Aperture Application Support folder. Moving it out of the way will force Aperture to generate a new one. You can also completely reinstall Aperture.

STEPS: The Aperture support folder is located at  ~/Library/Application Support/Aperture. Navigate to that folder in the Finder, and move it to the desktop (don’t throw it away). If the problem goes away, because there are so many things in this folder you may want to try putting pieces back one at a time. It’s tedious, but it could be worth your time so you don’t lose presets that you’ve built.

If you want to reinstall the app (although it’s rare this fixes the issue), you will need to get rid of the Preferences, the Cache, and the Aperture support folder first, then delete Aperture and reinstall it.

New User

IDEA: A classic OS X test is to create a new User account, and try to replicate the problem there. The individual user folders in OS X are very isolated, so if a problem exists in one, it’s virtually impossible for it to show up in another. If you create a new user account and the problem goes away, you know it’s in your User folder. If it’s still there, then it’s an OS-wide issue. By now you’ve eliminated any chance that Aperture itself is causing the problem, but you still need to figure out where it is hiding.

STEPS: Go to the menu > System Preferences… > Users & Groups. Unlock the pane (lower left corner) and create a new user account by clicking the (+) button. Make it an admin account so you have full privileges. Log out of your existing user account, log into the new one, and launch Aperture. It will create a new Library, but if you’re this far you’ve already proved that your Library isn’t the problem, so that’s OK. Run your tests and see where you’re at.

New OS or New Computer

This is a much more involved step, so if you got to this point and still have the issue, post a question in the forum first. However the first things that you will be advised is to test this on another computer if you can, and/or reinstall the operating system. If you’re on OS X Lion or later, read how to do that here. If you’re on an earlier OS, insert your OS DVD and launch the installer, and follow the on-screen instructions.