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Head Crash !

A definition of a head crash: When the heads of a hard disk come into contact with the rotating platters.

This, in theory means that many modern hard disks suffer a 'head crash' every time the heads park. However, there is a difference between the parking event and a head crash. When a hard disk parks it's heads, a command is sent to the drive to move the head stack assembly over the parking zone on the platters. As the motor loses speed, the heads gently come to rest in this are - the 'Parking Zone'. This area on the platter has a special lubricant embedded within the coating so as to minimise the friction between the heads and platters so that no damage is caused to the heads.

What Happens During a Head Crash

When the hard drive motor is spinning at full speed and for some reason, the heads make physical contact with the platter. At the moment of impact, one of two things will happen

1. No Platter Damage
   If the head crash is minor then the hard drive's shock detection systems will detect the sudden movement of the drive and attempt to move the heads to the 'safe zone' before any further physical damage occurs. Also, there are protective layers between the surface of the platter and the magnetic substrate that is hard enough to absorb the shock of a minor head crash therefore protecting the data. However, see the description for platter damage below as a head crash may have occurred but is not yet apparent. If you suspect this may be the case, power off the drive and contact a data recovery specialist.
   
   
2. Platter Damage
   If the head crash damages the protective layers and the magnetic substrate becomes compromised. This can quickly deteriorate into a full blown crash as the dust and debris particles displaced from the protective coating can increase the chances of subsequent head crashes as the drive has now become internally contaminated with this debris of which some may settle on the platter or become 'airborne' within the drive during power up. This can explain why in some circumstances a drive will function perfectly after a head crash, but then later completely fail. If there is any doubt if a drive has suffered a head crash, turn the power off and contact us.
   
   

Damage caused by a head crash

The picture below shows typical damage caused when the heads come into contact with the platters of a spinning drive.

The platters from a working drive have a mirror finish. It can clearly be seen that the heads have caused severe damage to the platters particularly close to the motor spindle, where most of the wear has occurred. This is caused by the damaged heads grinding into the platter as the drive was spinning. Power cycling the drive exacerbates this problem as each time the drive is powered up, more damage is done.

Generally, this type of damage is not recoverable, because the damage extends into the data area on the platters. In some cases where there has been only a minor head crash, some data is recoverable.

Causes of a Head Crash

A head crash can be caused by a number of things some are listed below.

• Dust or debris entering the drive. Hard disk drives should only be opened under a minimum of class 100 clear air environment. Opening a hard disk in uncontrolled environments can allow entry of dust particles. A single dust particle is all it takes to cause a head crash.
  
  
• Moving a computer while it is switched on. Definitely not recommended, a head crash happens in a fraction of a second and considering the average drive spins at around 7,200 rpm 1 second would equal 120 revolutions. By the time it is realised that a head crash has happened and the power turned off, the damage has been done.
  
  If you have to move your computer, even slightly, turn it off first. It is much better to have to wait a few minutes, than to risk losing all of your data.
  
  
• ElectroStatic Discharge (ESD). Static electricity is one of the biggest problems for manufacturers of all electronics equipment. A single static discharge can emit over 40,000 volts but at a very low current. This is enough for you to feel a small 'static' shock, but for delicate electronic equipment it is devastating.
  
  Some of the things that can happen to a hard disk that has been exposed to ESD are
  
  
  1. The electronics board fails
  2. The internal electronics (head preamplifier circuit) fails.
  3. The Giant Magneto Resistance (GMR) head fails.
  
  
  Items 1 and 2 are usually repairable, however it depends on the drive type, manufacturer and if spares parts can be sourced. item 3 above is the main one that causes head severe head crashes and platter damage. When a GMR is exposed to ESD it can often blow a small hole in it and sometimes even blow the whole GMR apart. These are what cause most head crashes. Prevention is possible by ensuring that the drive is fixed properly to the casing and that the casing is properly earthed. When handling computer equipment or working inside a computer use ESD safe procedures.
• Dropping a hard disk. Most likely to happen with external hard disks such as USB or Laptop computers. If the drive is spinning then expect serious damage, shock mechanisms do exist on a lot of modern hard disks, but few hard disks will survive a heavy fall when the drive is spinning.
  
  If the drive is not spinning when dropped, then you MAY be lucky.

How Can I prevent a head crash

1. Never move a computer while it is turned on
2. When working on a computer use ESD safe procedures.
3. Always make regular backups.

Eventually your hard disk will fail for one reason or another. My advice is backup your data, we back our main databases and accounts systems every 90 minutes and they are stored off-site. Every night, all of our servers are backup to tape.

See also the section on platters in our How is data stored on a hard disk section.