Why do my brakes squeal and how can I fix it?
First and foremost there is NO disc brake which NEVER squeals and NO disc brake type which ALWAYS squeals. Also there is no specific area where brakes have more problems. From super dry Scottsdale Arizona to wet Seattle there will be a few screaming brakes in every area of the country. Some brakes seem to have more problems than others but in general it's one of the following :
Dirty ( oily ) rotors - clean thoroughly with Isopropyl Alcohol
Dirty ( oily ) pads - clean with Isopropyl alcohol, then use emery paper and scuff the pads. Remember they need to be newly bedded in, so the first stops are not what you expect. If braking power is drastically reduced, replace pads with new ones.
Loose rotors and loose calipers - tighten them up.
Loose cones in the hub - again tighten them.
Loose spokes - sometimes you can detect this if you rub a little grease exactly where the spokes are crossing/touching each other. If the noise changes, then you need to retighten the spokes.
Loose quick-release - especially on alloy dropouts the q/r sometimes works itself into the alloy, the tension gets less and the axle starts vibrating.
Loose suspension bolts - seems to be a problem on the older I-drive bikes, but not only them. Check the owners manual for proper torques.
Loose Headset - adjust to specifications.
Loose .... loose... loose ANYTHING. Yes even a loose saddle can make a noise if vibrated by the brakes.
Incorrect alignment - use a Gnann-o-mat or Hope Spot tool to machine the tabs or use the CPS on Avid brakes.
But no matter what, some brakes just squeal. It seems like a lot of the problem comes from other parts of the bike where a harmonic vibration set up by the brakes manifests itself in an audible squeal. You have to work case by case, find the vibrating part and somehow cure it. I once had a squealing brake that was instantly cured by switching to a rotor from another manufacturer.