What Are Bad Sectors & Bad Clusters?

By Dan Stone

Bad sectors and clusters are parts of a storage device that aren't working.
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The terms "bad sector" and "bad cluster" refer to a specific section of a digital storage device that has been rendered unusable for reading and writing data. When a computer says it has found bad sectors or clusters it means that it has identified part of a connected storage medium that it can't access. The computer can attempt to recover data that's stored on bad sectors and clusters, but there's a chance the data is lost forever.

Sectors and Clusters

Sectors and clusters are ways of identifying and measuring data space on a storage device. A sector is the smallest storage unit on a memory device and a cluster is the smallest group of sectors the device can address at once. The storage device uses an allocation table to record what information is stored on which clusters. When the computer or computer-like device requests a file, it uses the allocation table to determine where the file is located on the storage device and then reads that section of the storage device. Different storage device formats support different maximum amounts of clusters on a device: clusters get larger as relative storage space increases. Files can't overlap clusters, so if a 16KB file is placed on a 32KB cluster the cluster's other 16KB is left unused.

Data Corruption

The data-writing process usually goes off without a hitch, but when there is a communication error or power loss the storage device may record inaccurate or incomplete data. When the computer or other device reads corrupted data it's not sure what to do with it or what to make of it. The file corruption may affect only the sector or the entire cluster depending on the severity of the error. If the corruption is not severe a computer may be able to recover all or part of the corrupted data.

Damaged Storage

Physical impact, repeated use and electrical failure can break part of a storage device's memory and render it unusable. Bad sectors and clusters born from hardware damage are typically permanent; however, instead of breaking the entire device, the storage device can mark the damaged sectors and clusters as bad units and just not use them for storage -- the difference reduces the device's capacity by just a few kilobytes. The data stored in a damaged sector or cluster might be recoverable if the computer reads over it several times.

Scanning and Fixing

Windows has two built-in tools that can identify and sometimes recover bad sectors and clusters. Within Windows, you can use the ScanDisk tool through File Explorer. Locate the hard drive in File Explorer, right-click on it, select properties, open the "Tools" tab and click "Check." ScanDisk will search the drive for bad sectors and clusters; if it finds any bad sections it will try to fix them. The Chkdsk tool works similarly to ScanDisk and can be accessed through the Windows "Action Center" or a DOS command-line prompt.