A Clicking Hard Drive
First, we need to differentiate between a few distinct types of hard drives sounds. There is the normal hum of the hard drive and the occasional increase of that hum when the system is being taxed (like when playing games or editing video). Over time, that smooth hum can sometimes start to sound like a faint series of very fast clicks, indication that the computer is thinking.
Louder clicks are usually not a good sign. If you turn on your computer and just hear a series of loud clicks, that is oftentimes indicative of a physical hard drive failure. This happens more often than we would like to believe; therefore, it’s always smart to regularly back up you important information. If your computer has loud clicks coming from the hard drive, chances are that you will not be able to load the operating system; although sometimes, loud sounds begin as the hard drive is in the process of dying.
Now, there is a third type of click that usually manifests itself when the Macbook is moved quickly when turned on or dropped. This is a single, fairly loud click. It is a normal sound for an Apple notebook and an important safety feature.
Apple uses a certain hard drive management in all of their portable notebooks called Sudden Motion Sensing (SMS) technology. Its purpose is to protect the hard drive from physical damage in the even that the notebook is dropped or hit hard. This is basically an accelerometer (like in the iPhone) that senses fast, jarring movements.
SMS is enabled by default in Mac OS X and when it is activated by a bump or drop, the user will hear a loud click from the hard drive. This is the sound of the hard drive heads being parked to prevent damage to the hard drive. Most physical hard drive failure occurs because the hard drive heads get damaged or set out of place. By parking the heads, the SMS technology reduces the risk of this happening.
When this occurs, usually there is no effect on the system’s stability. The only time the parking of a hard drive’s heads could cause issues is if the notebook is in the middle of doing something critical to the system, like booting up or installing a program. Usually, this is not the case.