Either the files are corrupted or you’re not running the version of ACR that you think you are. ACR 7.3 RC works with D600 files.
Assuming they are corrupted, how are you transferring the files from the camera to the hard-drive where LR is reading them from?
From discussion with others, it would appear that the files were corrupted during transfer.
I have some suggestions that I will try tomorow.
I have no problems opening old Nikon D700 NEF files in Photoshop CS6, and when ACR opens the file it indicates I'm using ACR 7.3 beta.
Nikon Transfer 1, which Nikon says is obsolete, will corrupt the headers of the newest Nikon camera raws, making them unrecognizable to Adobe software.
Tonight, I'll transfer the files from my SD card to the computer with Nikon view NX2 and see what happens?