You ran check disk on both the laptop and the external drive?
Yes (its my desktop that I am trying to backup)
Don't know if you still have this problem. It seems to be quite common with USB 3.0 drives and so far no-one has come up with a reason.
It may possibly be due to some obscure timing problem.
I was getting backup failures almost every time with my WD external USB 3.0 drive. After many months of searching, there was no fix but I did find a workaround.
I connected my external drive to my PC via a USB 3.0 hub. You can get them for under £10. Since then I have not had a single backup failure.