Break dust can also cause a squeek. There is a coating you can get at any auto parts store that prevents break squeeking. You apply it to the pads when installing them new. Its made for cars so I don't know the effect it would have on breaking if used on a cycle. So if you decide to try it be careful at first.
And as said above if it goes chirp...chirp...chirp it is probably a warped rotor.

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