A hard drive is a mechanical device. It consists of one or more platters that contain a material that can hold a magnetic charge (much like a cassette or VHS tape). There’s a magnetic drive head that can read and write magnetic data – it moves back and forth over the platters as they spin (typically at anywhere from 5400 to 15000 rotations per minute).

Inside a hard disk drive

As the platter spins, a cushion of air forms that hold the drive head over the platter so it doesn’t actually touch the platter. The head flies so close to the platter that even a dust particle wouldn’t fit between the head and the platter. In fact, if a dust particle did creep in there, it can cause the drive head to bounce and scratch the surface of the platter. That’s why hard drives are made in dust-free clean rooms and sealed.

Know what happens when you scratch a magnetic platter? You get some particles and dust. When those hit the head they can cause more bouncing. Which causes more dust… well, you get the idea. Next thing you know large stripes of magnetic material have been peeled off the platters, and with them – your precious data.

This is the worst kind of hard drive crash. Experts can sometimes recover data off of any undamaged portions, but it’s very expensive.

Drives can also experience other mechanical failures – usually a bearing failure (you’ll often hear the drive squealing in that case) or other mechanical error (you may hear a clicking). Experts can recover data from these drives by taking the undamaged platters and transferring them to another drive that is working properly. This is also expensive, but will usually be successful.

Drives can also experience electronic failures. Whether experts can recover data from these drives or not depends on the type of failure. Sometimes the failure will cause data to be scrambled on the drive, in which case you’re out of luck. But sometimes the data is unharmed and recovery can succeed (but again, it’s expensive).

If a hard drive is showing signs of failing, back up the data if possible right away and replace it (hopefully you’re making regular backups). If you can’t recover the data right away, stop using the drive. The data might be recoverable by experts, but if you’ve had a head crash, continuing to use the drive might will further damage and make successful recovery less likely.