Disc calipers are totally different, as they use absorbant pads. Squealing on a disc brake means that the pads are contaminated. There is not much you can do for this other than replace the pads, and when you do make sure you clean the disc rotor too otherwise you'll just contaminate your new pads.
Absorbant pads don't like anything with detergent in them, or lubricant (stating the obvious but you would be amazed at how many times I have had to replace pads on discs due to lube geting on them). I just use water and a clean cloth to clean mine, but you can get disc brake cleaner (made by x-lite) if you don't like using elbow grease.
. Watch out for most of these new 'bike cleaner' fluids, they may say 'disc friendly' on the label, but I am not a fan and always use just water.
I was at a race last week where part of the course went through a bar (great idea) and there was a DJ and lights and stuff (also a great idea) but then some bright spark decided to have a foam machine (bad idea). They created a wall of bubbles to ride through, which looked cool, but as soon as that stuff touched the discs it literally killed my pads. Came out of the bar, flew down the next part of the course, next corner, grab the brakes....uh-oh. Chewbacca piped up again and the performance of the brakes was non-existent. Luckily there was a none-to-friendly French rider who acted as my brakes instead
I realised afterwards that it was the detergent in the bubble stuff that contaminated my pads.
Some people say that if you boil the pads in water they get better, but this is a bit of a myth. Easy way to check is to run ur finger round the disc rotor, and if there is any greasy deposit on your finger then it is not good.
