What causes Bad Sectors on Hard Disk Drives (HDD)?

There are 2 categories of reasons.

1. You (nothing personal)

If you jar or jolt or jerk or jiggle or strike or shake or bang your pc while it is accessing the disk, you can damage that tiny piece, and cause a bad sector. And your pc is pretty much always accessing the disk, so…

What’s happening is that inside the hard disk drive is a…disk, an actual platter that is spinning fast, really fast. The platter contains all the data. As the platter spins the “head” of the disk (that which reads and writes the disk) floats over the platter on a laminar cushion of air flow with literally a few nanometers of space between them.  That’s about 1 millionth of a millimeter. So, yeah, small.

For an analogy, it’s like a 747 flying 1/32nd of an inch off the ground.

Sudden motions of the disk (or the computer containing the disk) can cause the head to contact the platter and scratch it. Usually this only happens for a fraction of a second and damages just a tiny portion of the disk. Just a sector or few.

2. Entropy

The theory of entropy states that everything in the universe tends towards disorder (or decay).

This is happening right now on your hard drive as you read this 😉

The specific sources of this decay are:

• original manufacturing defects
• heat
• wear over time
• tiny, tiny specs of dust (that get by the filter)
• vibrations of the building containing the device
• electrical disturbances (from the power company, lightening)
• error in the logic hardware of the drive
• “overclocking”

References:

• http://forums.overclockers.com.au/showthread.php?t=153022
• http://www.tomshardware.com/forum/265043-32-reasons-sectors

A “Bad Sector” is a term for a broken piece of Hard Disk Drive (HDD)–a very small piece. There are millions of sectors on a hard drive. EG, standard sector size is 512 bytes (aka 512B). Consider a typical disk (in 2015) of 1TB in size, or 1,000,000,000,000 bytes (1 trillion bytes). Doing some math: there are 2 million sectors on a 1TB hard drive.

That means everything you store on there is broken up into pieces of 512. Have a 100MB video file? It is stored as 2000 pieces of 512B each.

Fortunately, when sectors go bad, two mechanisms exist to help you.

1. The hard disk drive itself detects many bad sectors and remaps them so that data is stored elsewhere, in good sectors. You may never notice these.
2. Windows Operating System detects bad sectors and marks them to avoid using them in the future. Windows Vista, 7, 8 (and presumably 10 and later) detect when a “chkdsk” is necessary and prompt you to run it at next boot. If not, you can run “chkdsk” yourself.

Note: when the OS detects hard disk errors, you should IMMEDIATELY MAKE A BACKUP. DO NOT PASS GO. DO NOT COLLECT $200.

Then, get a new hard drive.

Usually things will only get worse. In the upcoming months, even days, even hours.

Why do sectors go bad?

Reference: https://en.wikipedia.org/wiki/Disk_sector