I found 2 techniques for silence squeaky disk (disc) brakes — all involving rotor cleaning. (If pads are contaminated they need to be replaced.)
The PinkBike reference claims “weak break in period” is the number 1 sin:
Brake squeal is most commonly caused by a weak initial break-in period, with the introduction of impurities like chain lube or chemical bike cleaners to the braking surfaces as a close second.
I bet I’m guilty.
break in your disc brakes with a series of very firm stops from a reasonably good speed before you get out on the trail. This properly deposits the pad material on the braking surface of the rotors. Typically, most riders drag their brakes lightly around every corner in the trail, as well as down easy descents. Lightly dragging a new set of brakes is a perfect recipe to create brake howl - which is why you see so many cross-country riders with noisy brakes.
Rule one: brake like you mean it, then let go.
The PinkBike reference is hard core. Clean with automative brake cleaner, Sand with 11 and 220 grit abrasive cloth (I’d try an orbital sander) then break it in properly.
I’m going to try the easy step first. The big job will wait until it gets really annoying…