A bad sector is a term used when one of the smallest units of storage on a computer hard drive is no longer usable. A bad sector means that one or many of the sectors on a drive can no longer be read from or written to by the computer. While a bad sector means that that small portion of the drive can no longer be used, it doesn't necessarily mean that the entire drive has failed.
The term originated on platter style storage disks, where a spinning, circular platter is read and written by heads that are capable of translating the magnetic fields on the platter into electrical currents. The disk's platter is broken down into different parts for storage, the smallest part being a sector which can contain data in sizes of 512 bytes and up, depending on the disk. The platter of a hard drive can be visualized as a circular racetrack. It is made up of many concentric rings, which are further divided into tiny segments called sectors where the data is stored. In the case of a bad sector, there is something of a pothole in the track that can no longer be driven over.
The causes of bad sectors are varied. When a bad sector occurs it is most likely due to an issue with the physical disk, as opposed to a software or operating system problem. The first place a person will likely hear about a bad sector is when his computer operating system software starts acting up.
It's normally not possible to repair a bad sector. The spinning platters eventually wear out, and so a few bad sectors are inevitable and are rarely even noticed by the average computer user. It's only when numerous bad sectors begin to appear that the drive is facing an imminent and complete failure.
Most computer operating systems now come standard with some sort of drive-monitoring software. To further help deal with the issue, disk drive manufacturers have also implemented software onto the drives that monitor the health of the disk. The Self-monitoring, Analysis and Reporting Technology (SMART) system that comes built into many hard drives can communicate with software on a computer that will alert the user when something is going wrong with a disk.
As solid-state media became increasingly popular, the term for a bad sector has made its way onto those types of storage media as well. With solid-state media such as flash memory, however, there are no tracks laid out on circular platters. Instead, a series of transistors works with the electronic signals. In this case, then, the bad sector moniker is being used to identify when a transistor as failed.