Just to be clear, I didnt put this in "mechanics" just because I figured more touring folks would see this....
will try to make this short (ish)

have had a disc brake bike now for about three years, and have had to learn, figure out, muck up, muddle my way through stuff with them, but all and all like how they work for loaded up touring, especially in mountains.

comes down to having to learn new techniques to work on them, and without getting into specific models and such (mine are tried and true Avid BB7's that work perfectly fine for me and while one piston, give me ample braking)
the one thing I figure any of us with discs will run into are noisey, screeching brakes at some point.

my bike started this on the front brake last year, and got worse and worse, and actually started with new pads I put in for my Mexico only trip, the Central America trip pads were getting a bit thin but figured I'd just change them given that I had spares.

The screech started out slowly, and would go away with slightly harder braking, but got worse and has been annoying for a long time now.

Figured it was contaminated pads, from having wheel off for air transit, and maybe getting something on the cardboard stopper I stuff inbetween the pads in transport.
I tried taking out pads, cleaning with alcohol etc, car disc brake cleaner, no change.
I tried sanding away the top layer of pad, no change again.
Cleaned rotors with alcohol also during all this, no help.

so kind of just lived with it until the other day
repeated the same stuff, no help
even took out my propane torch and cooked the pads, no help

but then saw this webpage that describes sanding away the ridges that form, and my front disc looked very much like this one, in the "before and after" photo, "figure 2"

https://www.bbinfinite.com/blogs/new...isc-brakes-now

I just used some metal sandpaper I had around, did it by hand, didnt even take the rotor off, but worked my way around with reasonably pressure, always going perpendicular to the streaks.
Tried a moderate amount of sanding, not that much, until they looked scuffed up, but didnt over do it either

and voila, after doing the prerequisite multiple hard brakings to transfer pad material to the rotor, it seemed ok.
Went for a loaded up ride yesterday to really test it out, and finally the screech is gone.

it actually makes sense, after each time of cleaning pads , sanding pads, I could feel even more with light braking , a fast "pulsing" going on (maybe more after cleaning cuz the pads were more "grabby" with fresh material exposed.)
and this pulsing is what sets up the high frequency resonating vibrations that create the screech , think how a violin bow going over a string makes it vibrate and creates the high sound, and violinists put resin on their bow strings to create "grabbiness"

so it would appear that as my rotors wore, and ended up with grooves in them , just like the photo shows, the sanding removed the ridges, and hence the vibration causing pulsing.

anyway, I just figure that more and more of us are using disc touring bikes, and its nice to be able to deal with stuff on your own, and to have a better understanding of fixes.

I wish I had figured this out last year, but there you go. I probably could have just sanded the rotor and not done any of the mucking around with the pads, and it would have fixed it.
Hope this helps some of you a bit.

ps, rotor thickness does need to kept an eye on, and a proper mechanic will have a gauge to check if your rotors are getting too thin, so be aware of this, especially with sanding going on. I will ride by a shop and have them check the thickness out again, did it last year, but will make sure.