I've got Tektro disc brakes and had this problem, was eventually cured by re-centering the carrier in the disk as the squeal was caused by vibration of the disc when the disc was being temporarily warped when the brakes were applied. It's a simple job to do by loosening the two bolts on the carrier, applying the brake and tightening the bolts in-turn until the carrier is centered. Check when stationary that the the brake disc does not move when the brake is applied and all should be well.
I'm pretty sure this has cured it, but at the same time I applied GT85 to the back of the pads (couldn't find my copper grease), so this may also have helped, time will tell.