I upgraded my Mac to Sierra 10.12.2 this morning and experienced the same problem with my files as you did Kim. I am using the the latest editions of View NX-1 and Nikon Transfer 2. To my mind, the problem is that the latest Mac upgrade is not compatible with the latest Nikon software.
As always seems to be the case, Nikon lags behind Mac upgrades, but incompatibility issues like this have heretofore not cropped up for me. Fortunately, the shots seem not to be lost, and I believe the complete NEF files are hiding behind these stripes because I opened them in Photoshop Bridge and the full files initially appeared but then the stripes soon appeared thereafter.
When those crack software support people at Nikon get it together to support Sierra 10.12.2 sometime in 2017, I'll wager these files will open just fine. By then, I probably will have completely stopped using Nikon software so compatibility will be moot. Iridient and Photoshop here I come!
I am beginning to really hate Nikon software.