Bad sector
A damaged disk sector causing data loss and system avoidance.
A bad sector is a section of a disk drive that cannot be read. Any data stored there is lost when the sector gets damaged. Once the operating system, such as Windows or Linux, identifies and marks a bad sector, it will avoid using that sector in the future. Bad sectors create a data security risk because they can leave behind recoverable information.
Bad sectors fall into two categories: soft (logical) and hard (physical). Soft bad sectors occur when the on-disk format becomes corrupted beyond what error-correcting code can fix, due to issues like power loss, bit rot (more common on floppy disks), or firmware problems. Writing over a soft bad sector can succeed. Hard bad sectors result from physical damage—such as head crashes, wear and tear, physical shock, or dust—and cannot be repaired; writing to them fails and forces the drive to remap the sector. New drives may have a few harmless bad sectors from manufacturing, while larger clusters appear over time. On solid-state drives, flash wear or controller errors can also cause bad sectors.
The operating system or the disk controller can detect bad sectors. Most file systems have a way to mark sectors as bad so the OS skips them. Diagnostic tools like CHKDSK (Windows), Disk Utility (macOS), or badblocks (Linux) can scan for bad sectors when a user requests it. With modern SMART-enabled disk controllers, the drive itself often handles avoiding bad sectors. Some newer file systems, such as Btrfs and ZFS, lack a built-in bad-block avoidance feature. Software that finds bad blocks can still be useful: by writing to a detected bad sector, the user can speed up the remapping process and prevent further read attempts.
When a sector fails a normal read, a typical post-1990 disk controller firmware will retry several times, hoping for success, before timing out and marking the sector as "pending." A successful read must pass error correction. Pending sectors may be retried on later reads, and the repeated retry noise is known as the click of death. If a sector is unwritable or cannot hold written data, the firmware remaps the logical sector to a different physical sector, regardless of whether it was pending. If a pending sector is successfully written to, it is removed from the pending list. These operations are invisible to the operating system, which only issues read and write commands.
- Percentage of drives with latent read er
- 3.5%
- Study duration months
- 32
- Number of drives studied
- 1530000
- Number of drive models
- 30
Lore & Background
Bad sectors are classified as either soft (logical) or hard (physical). Soft bad sectors arise from power loss, bit rot (more common on floppy disks), or firmware issues that corrupt the on-disk format beyond error-correcting code repair; writing over such corruption can succeed. Hard bad sectors result from physical damage such as head crashes, wear-and-tear, physical shock, or dust intrusion, and cannot be restored—writing to them fails, forcing a remap. New drives may have some innocuous bad sectors from manufacturing flaws, while larger patches develop over time. On solid-state drives, flash wear or flash controller errors can also cause bad sectors.
Operating systems and disk controllers detect bad sectors. Most file systems have provisions to mark sectors as bad, and diagnostic utilities like CHKDSK, Disk Utility, or badblocks can actively search for them. With SMART-enabled disk controllers, the burden of avoidance often falls to the disk itself. Some newer file systems such as Btrfs and ZFS lack a bad-block avoidance feature. Software tools that look for bad blocks can expedite remapping by issuing writes at detected bad sectors.
Disk controller firmware typically retries a failed read several times before marking the sector as pending. Pending sectors may be retried on further reads, and the repeated retry action produces a noise known as the click of death. When a sector is unwritable or fails to hold written data, the firmware remaps the logical sector to a different physical sector. Remapping is transparent to the operating system. Two types of remapping exist: P-LIST (factory production tests) and G-LIST (consumer usage). SMART information can report reallocated sectors and remaining spare sectors. Once the G-LIST is full, the drive shows write errors to the operating system.
Reader's Guide
The handling of bad sectors has evolved significantly, with modern disk controllers and operating systems managing them largely transparently. The SCSI command set offers finer-grained control than ATA, allowing users to read the G-LIST, control automatic remapping, and manually remap sectors. For sectors that fail the first read but succeed later, ATA provides ways to ignore or refresh the data but not to remap, leaving potential for further rot. Fully unreadable sectors demonstrate total data loss capability, and old filesystem mechanisms can avoid writing new data to such sectors. The SCSI/ATA Translation standard defines a read-write-verify sequence for translating commands. Historically, ATA and SCSI had commands for writing raw sector contents including ECC data, which could create deliberate soft bad sectors for testing disk utilities and forensic tools. The newer ATA command WRITE_UNCORRECTABLE flags a sector as bad immediately, with a SCSI equivalent available. The counterpart READ_LONG for reading raw sector contents is now obsolete. A study observed that 3.5% of drives developed latent read errors, and that disks with bad sectors are more likely to develop additional ones, with bad sectors clustering spatially and temporally. Errors recovered by ECC also suggest a higher chance of future bad sectors.
Did You Know?
- Bad sectors can be soft (logical) or hard (physical), with soft sectors potentially recoverable by overwriting.
- The repeated retry action on a pending sector produces a noise known as the click of death.
- SMART information can report how many sectors have been reallocated and how many spare sectors remain.
- A study found that 3.5% of drives developed latent read errors, and bad sectors cluster spatially and temporally.
Frequently Asked Questions
What is a Bad Sector?
A Bad Sector is a portion of a disk drive that has become unreadable, meaning any information previously written to that location is permanently gone. Think of it as a dead zone on the drive's surface that the system can no longer access.
What are the two main types of Bad Sectors?
They split into soft (logical) and hard (physical) varieties. Soft bad sectors arise when the on-disk formatting layer gets corrupted beyond what error-correcting codes can repair, often triggered by sudden power loss or gradual bit rot on older media like floppy disks. Hard bad sectors, by contrast, stem from actual physical damage to the storage medium itself.
How does the operating system handle a Bad Sector once it detects one?
Once an OS such as Windows or Linux flags the damaged area, it permanently routes around that sector so no future reads or writes target it. This keeps the rest of the drive operational but effectively shrinks the usable capacity.
Why do Bad Sectors pose a data-security risk?
Even after a sector is marked bad, residual traces or leftover fragments can sometimes still be recovered with specialized tools. This means sensitive information that was supposedly destroyed may actually persist on the platter, creating a vector for unintended data leakage.
How prevalent are Bad Sectors across the drive population?
A large-scale study tracking 1.53 million drives across 30 different models over 32 months found that roughly 3.5% of units exhibited latent read errors at some point. That figure underscores that even modern drives are not immune to developing unreadable sectors over time.
More in Computer Storage, Part 3 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced