Friday, January 11, 2008

Part 3: How Disks Fail

Now, we have to imagine how all the parts work together. The platters spin at reasonably high rates, anywhere from 3400 rpm (old drives) to 15000 rpm. The heads move at high speeds as well. The controller card maintains the speed of the platters and manages the movements of the heads. All the rest of the components are there to refine the signals and manage the communications.

head crash imageThe first and most obvious failure is the head crash. A head crash occurs when a head touches the platter and damages the media below. If you recall from the prior post there are several layers to the media. The last 2 layers are a hardening layer and a lubricating layer. Once the head has abraded through these 2 layers, the data layer is easy to damage. The last 2 layers are there for that reason to protect the magnetic layer. Heads can touch the media without damaging them. However, the heads are made out of the same materials that IC's are made from, glass and silicone. Glass is one of the hardest substances and can easily scratch most other materials. Sandpaper is made from glass. Considering how fast those heads are moving in relationship to the platters, it doesn't take much to scratch the media. A weird consequence of the hard crash is "stiction". Stiction is just as it sounds. It is occurs when the head sticks to the platter. Stiction occurs due to several factors including magnetic attraction, smoothness, electrostatic attraction and the stickiness of silicon.

The second common type of failure is electronic. Electronic failure is damage to any of the electrical components. The chain of components includes the control circuits, reading and writing circuits, and the communication circuits. These circuits can fail as a result of heat, cold solder joints, manufacturers defects, and externally generated failures (surges, physical forces, etc). It is interesting to note that an electronic failure can easily mimic any of the other failure even a head crash.

The last type of failure I will talk about is firmware failure. On each hard disk is an eprom that holds information and software that manages the functions of the drive. In addition some manufacturers put a part of the information and programming for the hard drive on the platter. This is commonly called the "firmware". We separate out this type of failure from electronic failure because it is addressed differently when we perform data recovery.

Here is how we describe the failures that can occur:

Head Crash with:

damaged heads
media damage

Electronic Failure with:

arc damage to the media
damage to the firmware

Next Post I will talk about software failures.



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