The old adage says there are only two certainties in life: death and taxes. I’d like to add a third: hard drives die. It might be today, tomorrow, next week, next year. It might be five years from now. At some point, it will die. The eternal question is, why? Hard disks are prone to two types of failures: physical and logical.
A standard hard drive is made up of three components: the platter, the head and the controller. The platter is a metallic disk coated with a magnetic surface where data is stored; each platter has two sides and most drives have more than one platter. The heads are similar to the needle on a record player — they float over the platters and read or write data as needed. The controller is just that — it controls the read/write heads and translates commands from the operating system to the drive itself. When you click the Finder icon, it sends a command to the controller saying “give me a list of folders.” It sends the necessary commands to the heads to gather this information, then sends it back to Finder.
If you’ve ever wondered what specs like “5400 RPM” or “7200 RPM” mean, it refers to how fast the platters are spinning — 5400, 7200 or even 10,000 rotations per minute. The faster the platters spin, the faster you can access your data. The platters are spun by a motor, and anything that spins that fast for a long enough period of time will eventually wear out. If the motor dies, the platters cannot spin, and you can’t get to your data. A sign that the motor might be failing is a loud buzzing or grinding noise from the hard drive as it spins.
Under normal operation, the read/write heads do not ever touch the platter. There is a gap of about 3 nanometers between the head and the surface of the platter. If you don’t know how small a nanometer is, picture a frisbee flying at a speed of 150mph about 3 inches from the ground. That’s how close to the platter the read/write head is, which doesn’t leave a lot of room for error.
Just as an errant gust of wind could cause the frisbee to hit the ground, any kind of shock to the hard drive can cause the heads to crash into the platter, scratching the surface and possibly causing the magnetic coating to be ruined. This can be anywhere from hardly noticeable to catastrophic, depending on where the heads happen to hit. Damaged areas can no longer reliably store data, but if there was already data there, it may now be inaccessible. Attempts to read damaged areas can cause the system to slow down, hang (beach ball) or freeze up altogether.
Condensation can also ruin a hard drive. Common belief is that a hard drive is completely sealed and air-tight, but this is not strictly accurate. There is a tiny air hole on the drive’s case (usually marked with a label that says “Do not cover”). Its purpose is to stabilize the pressure and humidity inside and outside the drive. If you leave a laptop in a cold car and then bring it into a warm room, you should let it sit at room temperature for several hours before trying to use it. Seagate drive packaging suggests that if the drive was shipped at a temperature of 30 degrees, the drive should sit for 15 hours before attempting to power it up. I have seen at least one system that was left in the car overnight and then would not boot from the drive the next day. The hard drive was unrecoverable; I can’t help but wonder, had the user waited until the system warmed up, if it would have been okay.
Modern hard drives will automatically move the heads off to the side, away from the platter’s data area, when you power down your system. You should always either shut down your laptop or put it to sleep before moving it to reduce the risk of a head crash. Carrying it around while it’s running is just asking for trouble, even with modern hard drives’ sudden motion sensor, which parks the heads when motion is sensed.
If you set your system down too hard (or drop it) while it’s not running, the hard drive will often be fine. If the system is running when it hits, the read/write heads can bounce off the platter and corrupt the data stored there, even with a sudden motion sensor. The platters have a lubricating coating that help protect against the casual bump here and there, but this won’t prevent the shock of a drop from damaging the drive.
The controller, like any electronics, can fail with or without any warning. Twenty years ago, hard drives and controllers were separate parts, so if the controller failed you could just swap it for another one and the data on the hard drive would be fine. Now the two are integrated, so if one fails the whole thing fails. Without the drive controller, the system can’t send commands to retrieve data. If you’ve ever turned on your computer and heard a clicking noise from your hard drive, this usually means the controller has failed and can’t send the correct commands to the read/write heads. As a result, they just knock back and forth. What would make the controller fail? Power spikes. An accidental drop. Liquid. Bad luck.
I haven’t mentioned the “b” word yet…anyone who’s read my articles knows I strongly recommend backing up your system regularly. Some day, your hard drive will develop problems. When it happens, one of the more difficult parts of my job is telling people that their data is gone. All the pictures of their baby growing up, all their college papers, all their music, gone forever. Your hard drive might die, but your data doesn’t have to go with it.
Back up your data. A technician’s least favorite job function is to report that all data is lost.
Originally written by Glen B. back in 2011. You can view the original newsletter here.