I was just burning a DataSafe 8x DVD+R disc which is CMC when Nero stopped half way through with a tracking error.

When I took the disc out you can clearly see the circle of data but at one side of it there is a smudged part.

Now, what I want to know is, is this a disc defect or a writer defect.

I used an LG 4163B writer.

Almost certainly, a disc defect.

What kind of a smudge? Dirt/oil/fingerprint on the surface? Or a dark spot in the purple dye inside the disc?

It takes a pretty large and/or dark dye spot to stop a burn. Here’s what a moderate-sized, 3-4 mm in diameter dye defect looks like in a quality scan:

Yup, I got a whole bunch of these on 6 packs of Maxell discs that I purchased a couple of months back. The thread is around here somewhere with the pics.