This happened to me twice - once the culprit was a bad card reader and I was ultimately able to get the files back using RescuePro (for SanDisk cards).
The second time, I, too, was in a hurry, and in hindsight, I don't think my MAC had completely booted. Those files were ruined, and I was never able to recover them. Going forward, I am much more patient and always wait for my MAC to fully boot before I attempt to load anything. (Knock on wood - no recurrence and it has been a year since it happened)
Good luck. This website has folks with a lot of knowledge. Hopefully one of the ideas they gave you will work.