In this day and age nearly everyone knows a little something about computing, whether it is how to download new songs to your iPod or how to download illegal content from your favorite torrent site. The population is generally more technically savvy then we were even three or four years ago. When the average person hears “Hard drive”, most actually know that there are at least two things involved in a hard drive. There are disks and there “needles” or “heads”. Many people think of hard drives as highly advanced record players, and in some aspects they are right. Just like it’s much older grandfather, the hard drive has both a record (platters) and a needle (heads). The platters store data by utilizing magnetization, and the head has the job of both reading these magnetic fields and changing them (what we call writing).

There are many components of the Head Stack Assembly, or HSA for short. These components include the actuator, sliders, gimbal, and the heads themselves. The heads themselves are so small that it requires a microscope to view them.

In the Above picture of a 2.5 inch Hitachi Travelstar we can clearly see many of the components of the HSA.

Shock absorption system

Starting on the far left side we have the “Shock absorption system”. This part’s primary function is obviously to absorb any shock that occurs when the heads load back on to the load/unload system (the ramp). This is the part that is notorious for making noise when you shake the drive itself, and many users incorrectly assume this sound means that there is a problem with the drive. The noise is actually normal; the shock absorption system fits very loosely inside.

Top Magnet

Moving to the right a little bit we see the top magnet. Normally there are at least two magnets in a hard drive; rarely are there less. In some recently manufactured low capacity drives you may find only one magnet, but traditionally you will always find a top and bottom magnet. These magnets are VERY powerful and require a significant amount of force to remove and very careful planning to put back on. Their purpose is to provide a magnetic field that will be utilized by the voice coil, which is nestled in between the magnets.

Voice Coil

The voice coil is made up of tightly wound copper wire. When a current is driven to the voice coil, it produces a magnetic field. This magnetic field reacts with the field produced by the magnets which causes a reaction. Because all of the magnets are affixed by screws, the associated magnetic fields stay in place. Because the voice coil is mounted to the head stack (which is movable), the entire voice coil has the ability to move away from the other magnetic field. By changing the current, the voice coil can be manipulated to move in multiple directions. This is how the heads are able to move, and move at extremely high speeds accurately.

Axis and Arm

The next components you will see clearly are the axis and the arm.

The axis allows the arms to move across the disk surface.

preamplifier + leads

If I could pick out any component of the head stack assembly (besides the heads themselves) as the second-most important part, I would pick the preamplifier. The frequencies of the signals that are coming from the head stack are very high (above 1ghz), so these frequencies would most certainly dissipate well before they could reach the MCU (Master control unit) without some preventative measures. Enter the preamp, whose job is to amplify the signal coming from the heads and send it out to the MCU. Its other duty is to control which head is to be used, and also if that head is to read or write. This is necessary because the nature of how the data is written and read from the platters is that only one head may be utilized at a time, and that one head can either read or write at any given time, not both. The preamplifier controls this function along with the MCU.

The Leads pictures above to the right of the preamplifier chip are simply the connections to the heads themselves. You can clearly see that this particular drive has 4 heads and there are 4 leads present. Sometimes the lead may be present, but the head is not in use. This can be easily identified as you will see that the lead does not continue down the arm to a head and it will end near the preamp.