Had this car (2006 A6) a few weeks, not driven much, but was driving fine. Suddenly today, after standing for a week with parking brake on, and when attempting to drive it there was a clunk sound on each rotation of the wheel. Found this line, about a 0.5mm tall ridge on one rear disc. Just curious what may suddenly cause such a thing? I haven't washed the car or done anything with it.
If you can take the rotor off and either sand or somehow remove the "line" of gunk, it should be just fine. More than likely it is rust build up which has been locked in place by pad adhesive. Some brake cleaner might take it off, but it's probably going to take some work to get it off completely. If that doesn't work, taking it to a shop and having a very light surfacing done on it will do it. There doesn't appear to be any real issues with the rotor, just the gunk. Once that's off, it should be just fine.