Try a different USB cable and or port on PC/Laptop
Make sure it has a ferite core around it (one of those elongated bulbs that clamp over the cable or are moulded into it.
Another option is also to check the software. is there an option to dedicate the USB port to downloads. I give it higher priority. This is what solved some read noise I was having on my guide camera.
One last thing is check power supply. Try a different one if you can and also make sure the power supply lead has a ferie core around it as well.
This is the only time it was happenned so far, so I am a bit puzzled. I will try again to image tonight and see if it was a one off. Otherwise, I will be trying some of the suggestions!