Errors loading some photos/videos from GoPro Hero 4.

For the most part I have had good luck using Aperture to manage photos and videos from my new GoPro Hero4. But today I attempted to upload a bunch of items and received an error message -- something like "an error occurred, the following 4 items could not be loaded." Retrying gave the same error.

My GoPro camera is connected to my iMac via a USB cable. Any insights on what this error is about and how I can resolve or work around it?