Bad Sectors? Learn More about them.

A bad sector is a small section of the hard drive that becomes damaged and can no longer have data written to it.  In most cases a bad sector is a relatively minor problem: while it can affect both traditional magnetic and newer solid state drives, both types of drive are designed with a certain amount of redundancy built in so that bad sectors do not cause data loss.  Problems arise when the number of bad sectors exceeds the amount of redundant space, causing the hard drive to fail and data to be lost.  The more bad sectors a hard drive has, the smaller the capacity of that drive gets, meaning that eventually the storage capacity of your hard drive can drop to a fraction of what it is supposed to be.

What is a bad sector on a hard drive?

Though both magnetic and solid state drives can be affected by bad sectors, in both types of drive there are two basic categories of bad sector.  A “hard” bad sector is one that is the result of physical damage, and is much more common in magnetic drives.  It could be the result of a small nick or scratch to the disk; it could be caused by the drive head meeting an internal stop; it could be the result of dust getting between the drive head and the disk.  In a solid state drive, the flash memory cells can become worn out after they are written to many times.  Unfortunately, hard bad sectors cannot be repaired because the area of damage is usually microscopically small; however, one or two bad sectors is quite normal in any frequently-used hard drive, and the drive can usually be used just as normal, though it’s always a good idea to have backups of your data in case more damage occurs.

A “soft” bad sector is the result of software or data error rather than physical damage.  Each sector of a hard drive generates an error-correcting code when data is written to or read from it.  If the error-correcting code is not consistent with the contents of the sector, the computer will mark that sector of the hard drive as damaged and stop writing data to it.  Soft bad sectors can develop because data writing to a sector is interrupted – perhaps by a power cutoff – because of a virus or malware, or as a result of software error.  Unlike hard bad sectors, soft bad sectors are usually easy to repair.

How to identify bad sectors

Hard drives are designed to self-diagnose bad sectors using S.M.A.R.T. – a technology that determines whether sectors are bad and whether the drive is in danger of failing.  Windows has no built-in system to read S.M.A.R.T. data, but there are plenty of downloadable programs designed to read and analyze the data so that you can determine whether the disk needs to be repaired.  Applications like CrystalDiskInfo can be set to run in the background during your computer’s normal operations so that you can be alerted to bad sectors as soon as they occur.  On a Mac, you can get basic information about a hard drive’s S.M.A.R.T. status using Disk Utility, or you can download SmartUtility for Mac for more detailed S.M.A.R.T. data analysis.  You can also check for bad sectors without reading S.M.A.R.T. data by running Windows’ built-in disk check system, also known as chkdsk.  Windows’ default is to run chkdsk whenever it recognizes any kind of disk error, but you can also run it manually if you’re worried that your hard drive is developing bad sectors.  Linux also has a built-in Disk Utility that allows you to look at your hard drive’s S.M.A.R.T. status.

What happens if your hard drive has bad sectors

Bad sectors are actually very common, and if your hard drive only has one or two they are nothing to worry about.  Almost all hard drives are built to have a little redundant space in case of bad sectors: if a computer recognizes a hard drive sector as bad, it will mark that sector, reallocate the data to another sector of the hard drive, and stop writing data to the bad sector.  It is still possible to lose some data when this happens: if individual files have become corrupted or are the cause of the bad sector, they won’t be copied over to the new sector, so you should have backups of your important files at all times.  But serious problems typically only occur if your hard drive develops multiple bad sectors.  If the hard drive runs out of extra space to reallocate data from the bad sectors, it can cause your storage capacity to drop, which is particularly a problem in solid state drives.  In addition, multiple bad sectors developing over a short period of time is usually a sign that there is a larger problem with the hard drive, and could mean that your drive is about to fail completely.  If this is the case, rather than trying to repair the drive, you should concentrate on making sure all your files are securely backed up.

How to repair bad hard drive sectors

Hard bad sectors cannot be repaired, but unless the drive has enough hard bad sectors to impact its performance, they are usually not a big problem.  Soft bad sectors, on the other hand, can be repaired by reformatting the individual sector to wipe the bad code and replace it with zeroes.  Most operating systems have a built-in system to repair bad sectors.  Chkdsk is designed to repair soft bad sectors as it finds them; so if you run it regularly it is unlikely that soft bad sectors will cause you a problem.  Mac and Linux allow you to repair soft bad sectors manually from their Disk Utility systems.  If your Disk Utility is still showing bad sectors after running the repair utilities, that is probably indicative of a larger problem with your hard drive.  In either case, it is worth taking your computer to be looked at by a professional technician so that if there is an underlying problem with your drive, it can be diagnosed and repaired.

by David Molnar

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