Aperture is a Database Management program. The files it opens are Aperture databases, which Aperture calls "Libraries" (inane, but marketers are convinced that no-one without a CS degree will buy a program with the word "database" associated with it). The Library files show in Finder, which is your Mac's file management program. They have the extension "aplibrary". So the first thing to do is search your system drive for files with the extension "aplibrary". You can do this from within Finder (put ".aplibrary" in the search field, and set the source to your system drive) or with OS X's handy Spotlight (click the magnifying glass icon on the left of the Menu Bar, and put ".aplibrary" in the search field that drops down).
How many Libraries do you have? Open each of them -- are your missing photos there?
Thank you very much for your clear instructions.
I opened finder but became confused when you say above 'set the source to open your system drive' because there was no such option. I clicked on all my files and in the end put in variations of ".aplibrary" but nothing came up. I also opened the spotlight but again, came up with nothing.
Rightly or wrongly I had expected to see something that said "system drive" or something similar. I really thought we were going to dig down deeper within the system itself. Looking on Finder it would appear to be skimming the surface. Apologies, if this may seem pretty stupid to you.
As far as I'm aware I have one main library. But I do have a number in my trash since trying to restore/recover deleted information from a number of tools that I have downloaded from the Internet. One of those libraries in trash can't open, it tries to rebuild but merely copies ie duplicates the seemingly empty file.
I remain convinced that the photographs are in there somewhere.
Again, I appreciate any help.