Upgrade aperture library sitting on external HD

I just updated Aperture 3.4.5 to 3.5.1; when opening the new version i was asked to upgrade my library. Since i relocated all master files to an external drive (using function "relocate master" i firstly quit aperture and re opened when holding down the option key. Now i selected my external library but realised that there are no images? However when i select the internal aperture library i can see roughly 6900 images.

You may have misunderstood, what relocating does.

You opened the library on your internal drive in Aperture, and then used "File > Relocate Originals" to move the original files to your external drive, right? Now your Aperture library on the internal drive is referenced. When you browse library on your internal drive, all images are still there - Aperture references the originals on your external drive. It knows,where they are, because it moved them there. Your library on the external drive has never been connected to these originals and so does not show you any images. Only the library that was open in Aperture, when you relocated the originals is connected to these originals.

When i look at the finder, external HD i can see all my images.

You let aperture relocate the images to this drive, outside the library, and so they are visible in the Finder. Be careful, not to delete or move them accidentally.

I noticed that on a backed up version before the upgrade to aperture 3.5.1 there is no aperture library.aplibrary file?

I don't know. Have you named your Aperture library differently? "aperture library.aplibrary" is the default name, when Aperture creates a default Aperture library, because it cannot find the current Aperture library. That can happen, when you open Aperture without the drive with the library connected, or after updating.

Also when i look at the finder for the aperture library file on my internal drive there are only a few pictures. So why does aperture recognise 6900 when there clearly are none?

When you relocated your original images, the images were moved, but the library with all the important database files remained. That is how it is supposed to work. Make the library smaller, by removing the Media, but keep the library with the important databases where it is. Your library on the internal drive has all information about the 6900 images - your ratings, keywords, albums, etc, and it has kept the references to the relocated images and thus knows how to find them. This will work, as long as you do not modify or move the referenced files. Leave them exactly where Aperture put them.

so, i just click on upgrade the library where aperture indicates 6900 images, correct

Yes, select the library that is showing the correct number of images and let it upgrade.

Before upgrading it is good practice to make a copy of the library for backup, just in case.

You' re welcome 🙂