I had the same problem. I think your PC is too fast for your external hard drive, i.e. you are using a USB2 port on your PC and your external hard drive is USB 1.1. To prove my theory I plugged my external drive into my PC via a USB 1.1 hub which I just happened to have lying around. This slowed the file transfer down and after that everything went perfectly. It was a bit of a pain, I thought my external drive unit was USB 2.0 but obviously not so. In fact the problem only showed itself when I was transferring large files, small ones managed to get across the link OK. Solution: get yourself a USB 1.1 hub, they are as cheap as chips.