Seagate and Maxtor-brand hard drives startup sound

[SNC]

Good day!

I have a question related to Seagate and Maxtor-brand hard drives. Both they make a short hum/beep sound on startup. I know this is a normal condition, but I wonder what's the reason of that sound. Other models do not make it. I know that there is no speaker in the hard drive, so it must be the motor or bearings, so what exactly is it?

Thanks

[pepe]

I think what you hear is the controller applying a constant (open loop) rotating magnetic field to the spindle to get it rotating. In this state there is no feedback from the BEMF, since the spindle speed is zero and close to zero, ergo there is no or very small BEMF signal which it can't use as am rpm feedback. As soon as the BEMF signal is high enough, it switches to a closed loop mode where the applied voltage is synchronized to the position of the spindle, in this stage the buzzing sound turns into a smooth acceleration.