Disc brakes on bicycles? It will never catch on…..lol.
They are too modern for me but when you have a squealing disc on a motorcycle, car or truck, take the pads out, clean up as you have and put something like copper slip/ceramic grease on the BACK of the pad. Sometimes when you're braking, the drag caused by the friction causes the pads to vibrate against the disc which in turn can cause the high-pitched squeal.

Worth a go, let us know if it works!