A bad sector is a section of a hard disk drive that is physically damaged and can no longer reliably store data. Bad sectors develop over time as a hard drive ages and can cause data loss and file corruption. Here are some key things to know about the bad sector problem in hard disks:
What causes bad sectors?
There are several potential causes of bad sectors:
- Physical damage to the disk surface – This can happen due to a head crash, vibration/shock, wear and tear over time, etc.
- Manufacturing defects – Imperfections in the physical disk media that cause areas to fail.
- Electrical issues – Power surges or fluctuations that disrupt the magnetic storage.
- Corrosion – Oxidation or material breakdown on the platter surface.
- Overheating – Excessive heat beyond disk specifications that damages components.
How do bad sectors develop?
Bad sectors tend to develop gradually over time as a natural part of a hard drive’s aging process. Here are some of the typical stages of failure:
- Weak sectors – Areas that are beginning to fail but can still be read. May take longer than normal to access.
- Pending sectors – Unstable areas that have some recoverable data but also have corrupted data. Marked by the hard drive as suspicious.
- Reallocated sectors – Failed sectors that have been marked by the drive as unusable. Data has been copied to a spare sector.
- Damaged sectors – Failed sectors that cannot be reallocated or accessed at all. Typically the final failure state.
So in summary, bad sectors often begin degrading slowly over time before final failure occurs. The drive will attempt to reallocate data to mask some failures, but will be unable to hide them all.
What are the effects of bad sectors?
Bad sectors can lead to a number of problems, including:
- Data loss or corruption – As sectors fail, data stored on them can be lost or damaged. This leads to files that are unreadable.
- Difficulty writing data – Attempts to write data to bad sectors may be unsuccessful. This can freeze processes that expect data to be written.
- Performance slowdowns – Your hard drive uses extra effort to reallocate data from bad sectors, reducing overall performance.
- Difficulty accessing files – Metadadata stored on bad sectors may make it hard to find or open files stored elsewhere on the drive.
- System crashes – As corrupted data is accessed, it can cause operating system crashes and lockups.
- Difficulty partitioning – Bad sectors may prevent your operating system from partitioning or formatting part of the disk.
How can I tell if I have bad sectors?
Here are some signs that may indicate you have bad sectors developing on your hard disk:
- Frequent hard drive errors showing up in system logs.
- Windows or other OS freezing temporarily when trying to access certain files.
- Corrupted looking jpg images, mp3s, or other media files.
- Antivirus software flagging harmless files as corrupted.
- Recurring Blue Screen of Death (BSOD) crashes, especially with UNEXPECTED_STORE_EXCEPTION error messages.
- Standard diagnostics like CHKDSK reporting file system errors or bad sectors when scanning.
- Unusual noises from the hard drive like clicks, buzzing, or grinding.
- Slow performance and very long load times for files and applications.
Tools like Windows’ Error Checking utility, DiskProbe, or hard drive Sentinel can scan for and report bad sectors. Most hard drives also have S.M.A.R.T. capability to monitor conditions like reallocated sectors. Third party SMART monitoring tools can read this info to check drive health.
Can bad sectors be fixed or repaired?
True physical bad sectors cannot be repaired, since the surface or magnetic media is physically damaged. However, it is possible to remap them using spare sectors set aside for this purpose. The hard drive will attempt to reallocate files automatically, but you can also manually initiate a scan and repair process if you notice bad sector problems.
- Run CHKDSK /R in a command prompt to scan drive and repair file system errors.
- Use the Error Checking tool in Explorer to scan and quarantine bad sectors.
- Use the SFC /scannow command to repair system file corruption.
- Use fsck to check and repair the filesystem.
- Use badblocks to identify bad sectors and remap to spares if supported.
- Use dd to overwrite troublesome areas and force reallocation.
- Boot into Recovery mode and run Disk Utility’s First Aid verify/repair.
- Use diskutil from Terminal to verify and repair the disk.
This scanning can help control the problem, but physical bad sectors will usually continue to spread across a degrading drive over time.
How to avoid and prevent bad sectors
You can take measures to reduce the chance of bad sectors occurring and minimize their impact when they do occur:
- Use disk monitoring – Use S.M.A.R.T. tools to monitor disk health and catch issues early.
- Manage disk temperature – Keep your hard drive cool with airflow/fans to prevent overheating.
- Update firmware – Keep your disk firmware updated for potential error corrections.
- Handle with care – Avoid physical shocks/damage to disks from drops or vibration.
- Maintain file copies – Keep backups of important files to minimize data risks.
- Scan regularly – Periodically run disk scans to identify problems.
- Upgrade when needed – Replace older drives that are accumulating bad sectors.
Can lost data be recovered from bad sectors?
If the sector damage is not complete, it may be possible to recover data from sectors that are behaving erratically. This requires special data recovery software capable of repeatedly reading the sectors and extracting whatever data fragments can still be accessed.
Examples of tools that can potentially recover some data from bad sectors include:
- SpinRite – Assesses trouble spots, repairs data if possible, and copies any recoverable data.
- DMDE – Scans sectors and extracts accessible data fragments from problematic areas.
- Disk Drill – Scans and recovers lost files due to bad sectors or deletions.
- ReclaiMe – File recovery software designed for bad sector situations.
The effectiveness depends on the severity of the disk damage. If the platters or magnetic media are physically degraded, no software can reliably read the data. But specialized recovery tools offer the best chance of salvaging data from failing sectors.
Can a hard drive still work with bad sectors?
A drive with some bad sectors can still function and boot into an operating system. Modern hard drives reserve spare sectors that get swapped in transparently when an OS tries writing to bad sectors. However, performance and data integrity become progressively compromised as the number of bad sectors grows.
A general guideline is that up to 100 reallocated sectors is within acceptable tolerances for a consumer hard drive. But once reallocated sectors reach the 1000+ range, it becomes risky to continue relying on the drive. At that point it should be replaced.
Here are some typical scenarios that could occur with bad sector situations:
- A few bad sectors – Likely goes unnoticed outside of S.M.A.R.T. reports. Minimal effect on performance or reliability.
- 100-200 bad sectors – Some minor performance hit. Higher risk of file corruption. Monitoring recommended.
- 500+ bad sectors – Noticeable system slowdowns and potential freezing. Data loss becomes likely. Drive replacement recommended.
- 2000+ bad sectors – Frequent crashes, freezes, and file read/write failures. Drive failure looming. Data is at serious risk without replacement.
Once bad sectors exceed 10-15% of the total drive sectors, catastrophic failure is typically unavoidable. The drive may continue limping along until the spare sector pool is exhausted too.
Bad sectors are an inevitable side effect of hard drive aging. But being aware of their causes, minimizing risks, monitoring drive health, and having data backups can reduce problems. Tools are available to recover some data and temporarily prolong failing drives. Ultimately though, bad sectors multiply until drives fail completely. At that point, drive replacement is the only option.
|Bad Sector Stage
|Longer access times, but data still readable
|Corrupted data, but drive can recover some data
|Transparent remapping used, reduced performance
|Complete failure, inaccessible
Summary of key bad sector facts
- Causes include physical damage, defects, electrical issues, corrosion, overheating.
- Bad sectors develop through weak, pending, reallocated states before complete failure.
- Effects include data loss, performance slowdowns, crashes, trouble partitioning.
- Repair involves remapping via spare sectors, but doesn’t fix underlying problem.
- Preventive measures include SMART monitoring, proper physical handling, scans, and firmware updates.
- Up to 100 reallocated sectors is tolerable, but 1000+ indicates a dying drive.
- Data recovery software can potentially extract data from some bad sectors before complete failure.