If you select several images in Aperture and send them all to an external editor simultaneously, you might get an alert message that says some of the images could not be located. This can happen if you've selected several larger images and Aperture hasn't finished loading them all.
When you send a file to an external editor, Aperture must first create an intermediary file—a TIFF or PSD file, as determined by your preferences. If you select several large images at once, it can cause a brief caching delay—you'll see a "Loading Image..." message near the top of the Viewer when this occurs. If you try to send the files to an external editor before the images are done loading, Aperture won't be able to create the intermediary files and thus, the images can't be found.
To avoid this issue, wait until Aperture finishes loading all images (when the "Loading Image..." message has disappeared), then send your images to the external editor.
Note: Because Aperture has a limit on how much image data can be simultaneously cached, you may still get the message after all images have loaded. In this case, send fewer images at a time. Aperture does not have a set limit to the number of images that can be cached due to variations in image size and dependency on your computer's specific configuration.