Specialized Hardrock - How to stop disc brakes from squealing?
In my experience disc break squeal is typically a result of excessive vibration caused by one of three things:


Contamination - Brake disc and pads should only be cleaned with isopropyl alcohol.  Give them a thorough wipe down.
Poor adjustment/loose components - Try looking at the manufacturers web site for adjustment and torque specifications.  Installing and adjusting brakes is one place where a torque wrench can be helpful.
Lower end brake components - A lot of lower end disc brake sets have variations in the pads and rotors that just naturally encourage a squeal.  Sometimes as they wear it will diminish, but often you are stuck with the sound unless you want to fork out the money to upgrade.


Note that especially in case 2 and 3 you will see the behavior you describe where the squeal is worse when the brakes are applied lightly.  As you apply more force you remove the ability of the involved parts to vibrate.