HDD Brushless Motor runs jerky!
In order for it to turn smoothly, a brushless motor must have its winding current switched when the rotor is in a certain position relative to the windings.  If the motor passes this point without the windings being switched, the windings will start trying to impede the motor's rotation.  If the windings are switched slightly too early, they will impede the motor's rotation until it manages to coast to the proper point, whereupon they will start assisting it.  If they are switched too much too early, they will impede the motor's rotation so much that it doesn't reach a point where they would help, and the motor will stall.

If the windings spend any significant amount of time impeding the motor's rotation rather than assisting it, rotation will be jerky, and the fraction of the motor's power consumption which is spent driving the mechanical load rather than heating the motor will be reduced.  In order to make a brushless motor rotate smoothly and efficiently, it is generally necessary to sense its rotational position and ensure that the windings are switched at the proper times.

Note that one could make a brushless motor turn smoothly at any speed, even without using positional feedback, by driving its inputs with three-phase simusoidal current- or voltage waveforms, since the position toward which the motor would be trying to move the rotor would move smoothly, but such approaches should generally only be used in cases where one is driving the motor with a tiny fraction of its rated load.  The problem with such approaches is that the amount of voltage and current required for the motor to maintain a particular speed will vary depending upon the amount of torque it has to drive, and any power which is fed into the motor and not converted into mechanical energy will be converted into heat.  Unlike stepper motors which are generally designed so they can safely dissipate 100% of the energy put into them as heat, brushless motors are not.  Putting significant excess power into a brushless motor will destroy it in short order.