You have encountered a hardware malfunction that causes these colored bands.
It could be a bad camera card, or card reader, or USB cable/USB port, or bad hard disk, or bad memory. You need to check these components to find where the problem is.
Nikon Transfer displays the embedded JPG, not the whole RAW photo, and the embedded JPG is less susceptible to this corruption. So the fact that Nikno Transfer displays the photo without these bands is meaningless.
If you are not using an up to date version of Nikon Transfer, older versions of the software can do bad things to files from newer cameras. My recommendation would be to ignore Nikon Transfer and use LR or another utility to copy the files off the memory cards (and use a card reader to do that).
If you still have the original files on the memory card(s), try doing the transfer without Nikon Transfer.
Yes, it could be a hardware problem as Paige points out, but I would look to the software for a problem first.
If you have an older version of Nikon Transfer, it can corrupt your raw photos: Older versions of Nikon Transfer will corrupt D600 files. But if you have an up-to-date version, then dj_paige's diagnosis is the only explanation.
John and Joe, isn't it true that the "corruption" caused by Nikon transfer makes the photos unreadable by Lightroom? I am not aware that it will cause the corruption that looks like "rainbow bars of colour across them" as the original poster described. I think that hardware malfunction is still the culprit.
isn't it true that the "corruption" caused by Nikon transfer makes the photos unreadable by Lightroom?
That's what I've read too. I agree that bad hardware is the more likely cause. But given the history with Nikon Transfer, I think it makes sense to check its version as well, just in case.
... but but but
If the file is corrupted in such a way that the image displays "rainbow bars of colors", those pixels of the RAW image can't be recovered simply by the fix for Nikon Transfer issues. Those pixels are gone, unless the original raw photo can be transferred via different hardware in such a way that the pixels are uncorrupted. The fix for Nikon Transfer issues only corrects header information, not the pixel information.