What is bad sector check?

A bad sector check, also known as hard drive surface scan or bad block scan, is a diagnostic test performed on computer hard disk drives to detect bad sectors. Bad sectors are areas on the hard disk platter that can no longer reliably store and retrieve data due to physical damage or manufacturing defects.

Table of Contents

What causes bad sectors?

There are several potential causes of bad sectors:

Physical damage – Scratches, dents, wear and tear over time can damage the platters.

Overheating – Excessive heat can warp or melt the platter surface.
Manufacturing defects – Imperfections in the platter surface during manufacturing.
Age – Older hard drives are more prone to developing bad sectors.

Magnetism – Exposure to magnetic fields can alter data on the platter surface.
Firmware bugs – Bugs in the hard drive firmware can incorrectly tag sectors as bad.

How are bad sectors detected?

There are two main ways bad sectors are detected on a hard drive:

1. During a read/write operation

When the hard drive is unable to read or write data from a particular sector, the hard drive firmware will mark that sector as “bad” so it is no longer used for storage. This will trigger an error message such as “S.M.A.R.T. Status Bad” or “Uncorrectable Sector”.

2. During a bad sector scan

Tools like CHKDSK in Windows or fsck in Linux can perform a complete scan of the hard drive platters, analyzing each sector for errors. Any sectors that fail this scan are marked as bad by the OS.

Third-party tools like HD Tune Pro also allow manually scanning for bad sectors across the entire hard drive.

Why is it important to detect bad sectors?

Detecting bad sectors is critical for several reasons:

Prevent data loss – Bad sectors can lead to irrecoverable data loss if used for storage.
Avoid disk errors – Marking bad sectors prevents write failures and crashed programs.

Monitor drive health – Growth in bad sectors indicates potential hard drive failure.
Reallocate sectors – The hard drive can remap bad sectors to spare good sectors.

Finding and marking bad sectors helps maximize hard drive lifespan and reliability.

What happens when a sector goes bad?

Here are the typical steps that occur when the hard drive determines a sector has gone bad:

The hard drive firmware detects read/write errors from the sector.
The sector is marked as “reallocated” and taken out of use.

A spare sector is substituted from a reserved pool of spares.
Data is rewritten to the spare sector when the OS performs a write.
The original bad sector is no longer visible to the operating system.

This reallocation process helps mask bad sectors from the operating system to prevent crashes and data loss.

Can bad sectors be repaired or recovered?

Unfortunately, there is no way to truly repair a bad sector once it occurs. The physical damage to the platter surface is permanent.

However, the data contained in a newly developed bad sector can sometimes be recovered using forensic data recovery techniques. This involves specialized tools that can read and extract data from sectors even with physical damage.

But once a bad sector is reallocated, the original data is typically lost for good. The best way to avoid bad sector data loss is frequent backups.

How to check for bad sectors in Windows?

Windows provides built-in tools to scan for and attempt to repair bad sectors:

Using CHKDSK

CHKDSK scans the file system for disk errors and bad sectors. To run in Windows:

Open the Command Prompt as Administrator.
Type “chkdsk C: /f” and press Enter to scan drive C (replace C: with the drive to scan).
The “/f” parameter tells CHKDSK to fix any found errors.

CHKDSK will display a report of any bad sectors found.

Using SFC

The System File Checker can scan system files for errors and replace corrupted files:

Open Command Prompt as Administrator.

Type “sfc /scannow” and press Enter.
SFC will scan Windows system files and attempt repairs.

Using Diskpart

Diskpart is another command-line tool that can scan drives for errors:

Open Command Prompt as Administrator.
Type “diskpart”
Type “select disk X” (where X is the drive number to scan)

Type “detail disk” to list detected errors.

How to check for bad sectors in Mac OS X?

Mac OS X provides built-in tools such as Disk Utility and fsck to detect and repair bad sectors.

Using Disk Utility

Open Disk Utility

Select the drive to check
Click First Aid tab
Click “Repair Disk” to scan and repair errors

Using fsck in Terminal

Open Terminal
Type “sudo fsck -fy” to do a full scan on the startup drive
Type “sudo fsck -fy /dev/sdX” to check a non-startup drive (replace X)

fsck will display a report of bad sectors found

How to check for bad sectors in Linux?

Linux provides the fsck tool to scan for and repair bad sectors.

Using fsck

The main syntax options for fsck are:

```
fsck
```
– Interactive scan on startup drive
```
fsck -a /dev/sdX
```
– Scan specific drive sdX
```
fsck -y /dev/sdX
```
– Automatic repair without prompts

So to do a full automatic scan and repair on sda:

sudo fsck -fy /dev/sda

Preventing bad sectors

While bad sectors are sometimes unavoidable, these tips can help minimize their occurrence:

Use SSDs instead of HDDs – No moving parts so less wear.

Avoid physical damage – Handle hard drives carefully.
Maintain cool temperatures – Use cooling or ventilation.
Ensure proper shutdowns – Don’t interrupt power suddenly.

Stay away from magnets – Keep devices away.
Scan drives regularly – Run occasional surface checks.

Can bad sectors spread or increase over time?

Unfortunately, yes – as a hard drive ages, bad sectors tend to propagate and grow in number. This is because:

Nearby sectors may also be weakened or damaged
Magnetic particle buildup degrades media surface
Increased friction and heat from start/stops

Failed sectors stress remaining sectors

Most modern hard drives reserve spare sectors to reallocate in place of bad sectors. But once these spares are exhausted, the risk of data loss increases over time.

When to be concerned about bad sectors?

As a general guideline:

Less than 100 – Minimal concern on modern large drives
100-300 – Monitor drive closely, back up data
Over 300 – High risk of failure, replace drive ASAP

Also be more concerned if bad sectors are rapidly increasing week over week. The total count matters less than the rate of new bad sector growth.

Can you use a hard drive with bad sectors?

Drives with some bad sectors can still safely operate thanks to sector reallocation. However, performance may suffer as the drive relies more on reallocated spares.

As a precaution, avoid storing important data on drives with known bad sectors. Make sure valuable data is backed up elsewhere. Monitor SMART parameters for any indication the drive is failing.

While using a drive with bad sectors is possible, it’s recommended to replace the drive as soon as feasible to maximize data integrity.

How to recover data from a drive with bad sectors?

If the drive is still partially accessible, data may be recoverable using:

Data recovery software – Tools like SpinRite can read damaged media.

Advanced file copy – Robocopy, DDRescue can isolate readable sectors.
Drive imager – Create complete drive image to extract files.
Professional recovery – Labs can rebuild drive and extract data.

But preventing bad sectors in the first place via backups is much less costly than attempting recovery.

Can you fix bad sectors? Myths vs facts

There are many myths surrounding DIY bad sector repairs. Here are some common ones:

Myth: You can repair bad sectors with software

Fact: No software tool can truly fix a bad sector. Only a professional DR lab can attempt physical repair.

Myth: Low-level formatting removes bad sectors

Fact: Low-level formatting only resets drive state, it cannot repair physical platter damage.

Myth: Bad sector redirection is permanent

Fact: Reallocated sectors may fail later. There is no guarantee the redirection is permanent.

Myth: Drives stop using bad sectors

Fact: Drives will reuse marked sectors once spares run low, risking further issues.

While software may claim to fix or repair bad sectors, only replacing the drive can truly resolve the problem.

Conclusion

Bad sectors are unavoidable over the life of a hard drive. But running regular surface scans, monitoring SMART parameters, and promptly replacing failing drives can minimize potential data loss.

Treating drives with known bad sectors with care and ensuring proper backups are critical to safeguarding important data against unexpected failure.

While bad sectors cannot be permanently repaired, steps like reallocation, isolation, and redirection can temporarily mask issues – but replacement is the true solution.