What was this DIG (or may be D G ) Interface on IDE (PATA) CD Writer?

Digital Out. This is how it's labeled on my CD writer:

Google "cd spdif 2-pin" and you'll find suitable cables.

It is digital out. The digital auto out has both channels encoded and passed along one wire pair.

Not sure why it is labeled D/G--perhaps so you don't think you can connect it to an analog receiver--but it is signal and ground. The analog audio connector requires a pair for each channel, and it is labeled Right, Ground, Ground, Left. In reality, most analog audio connectors I have seen use a shared wire for ground so the connectors usually only have 3 wires.

Digital audio output for connecting to a sound card. Back in the day it (or the analogue equivalent) was required to play CD audio through computer speakers