How is your Drobo connected, Mark? Is it a network storage? Or directly connected by USB?
If it is a NAS, I'd move the Aperture library to a directly connected drive with a filesystem compatible with Aperture, MacOS X EXtended (Journaled).
All my images for the book are located within the Aperture library.
We have occasionally seen broken references to managed files in the Aperture Library.
These are difficult to find, since Aperture has filed them as "managed" and searching for "referenced" files will not turn them up.
Try to create a smart album and search with the rules
Any of the following that match
- File status: Offline or
- File status: Missing
Does that turn up any images with a broken arrow badge or yellow warning triangle?
If you cannot find problematic images this way, try to export the originals of the images used in the book. Missing images cannot be exported, so if you hit a broken images when trying to export, you will know you have found the culprit.
To try this systematically, select half of the set of images. If you can export them, you will know they are o.k. and the missing one is among the other half. Then export half of the other half, etc.