I've got a video I shot @ last years 12hour MTB event @ Werribee, it was on a switrchback corner just after the start with hundreds of bikes rolling through. All you can here is the multiple squeals of everyones disk brakes coming into the corner.
So what I'm saying is that you may be able to get rid of disk brake squealling in the short term with sanding the pads back alittle or realigning the caliper (brake type depending), but it'll come back if the conditions are right, like dry dusty riding or muddy going.
My Avid Juicy 5s on the Anthem 1 squeal every now & again, but there doesn't seem to be any ryhme or reason to it, so I just forget about it in the end.
Just a bike & a place to ride.