When I restart Aperture it is continuously busy 'processing' unless I rebuild the library each time; that's time consuming and annoying. I think I probably have one (or more) somewhat corrupted images and I liek to isolate it/them. I though one way to try would be to save e.g. projects somewhere else and then delete from Aperture; repeat until it's OK. Maybe there are better solutions? If not, how do I save parts of the library and bring them back later after finding they are OK?
But a question: what do I lose by not having the previews generated? Is that essential? One a quick test on images without regenerating the priviews I don't notice any differences.
Sorry, I did not have time to come back to you earlier :
The previews are needed to share your Aperture images with other applications. This is enabled in the Aperture Preferences -> Previews tab. Here you specify if you want previews to be generated automatically and the quality of the previews.
Previews are used by the Desktop and Screensaver, by iTunes to synch your albums with your mobile devices, by the image browser in "Mail", by the Media Browser in all iApps. Without previews you cannot browse your Aperture images from other Applications, you will always have to export the images. But if you do not need them, turn the generation of - that will save space and time.
You probably do not notice any difference right now, because most of your images have Previews. To test this behaviour open "System Preferences" and try to set your desktop wallpaper to one of your new images without Previews. Probably you will not see these images in "Desktop & Screensaver"