Bad sectors are a common underlying cause of data loss in storage devices. They are commonly associated with hard disk drives (HDDs) only, though conceptually, they are equally relevant to solid state drives (SSDs) and other forms of digital storage.
In one of the largest studies ever conducted, researchers monitored 1.53 million hard drives across 30 models over a 32-month period. They found that 3.5% of all drives developed unreadable bad sectors, and more importantly, they found that these sectors tend to appear in clusters. So, if your drive develops even one bad sector, the chance of the drive developing more bad sectors goes up sharply.
This guide is designed to help you understand exactly what a bad sector in a hard disk means, why sectors fail, and how modern storage devices handle these failures.
We will also explain what bad sectors mean for your data, in which cases bad sectors mean data loss, and whether/how your data can be recovered from a storage medium with bad sectors.
What Is a Bad Sector?
First, let’s understand what a “sector” means.
A sector is the smallest physical recording unit on a hard disk drive (HDD). It serves as the fundamental “chunk” in which data is organized, stored, and retrieved. In an HDD, sectors are laid out along concentric tracks on the disk’s platters.
Traditionally, sectors are 512 bytes in size, but in many modern high-capacity HDDs, a sector is 4,096 bytes (4 KB). So, sectors function as fixed “addresses” on the storage map of an HDD and allow the OS to read and write data precisely on it.
For a complete anatomy of how a hard disk is constructed, see our dedicated knowledge base on HDD design.
The concept of a sector extends to SSDs as well.
In the underlying NAND flash memory of an SSD, the smallest unit that can be programmed or read is a “page,” which is significantly larger than an HDD sector and ranges from 2 KiB to 16 KiB.
In NOR flash
memory, the term "sector" is sometimes used interchangeably with "block" to describe an erase segment, which is the smallest physical unit that can be erased.
So, when we say “SSD sector,” we technically mean a page or a block.
What Is a Bad Sector on an HDD?
A bad sector on a hard disk is a specific sector that has become unreadable or inaccessible to the operating system. This happens when the drive’s internal error correction systems can no longer reliably retrieve or update the data stored at that sector. This also implies that any information in that sector is at risk of permanent loss.
Bad sectors on an HDD can develop because of many causes, such as when a microscopic particle or scratch disrupts the tightly packed magnetic domains used to represent data. Also, over time, the ability of the disk to hold a stable magnetic field can degrade, leading to “bit rot.”
HDDs are sensitive mechatronic systems that operate at sub-micron tolerances. So, if the read/write head is even slightly misaligned, or if a platter suffers a “head crash,” the corresponding sector(s) can be permanently damaged.
Having hard drive bad sector errors doesn’t mean the entire drive is unusable. Most of the disk may still function correctly. However, even one bad sector on a hard drive can lead to file corruption or unexpected data loss. More than that, it’s a warning sign that the HDD may be degrading.
Bad Sectors in Solid State Drives (SSDs)
While bad sectors on SSDs are often called “bad blocks,” the underlying risk is similar; that is, a part of the storage medium can no longer reliably store or retrieve data.
SSDs use NAND flash memory, which stores data as electrical charges in floating-gate transistors. Over time, due to repeated program/erase (P/E) cycles, the tunnel oxide layer that keeps electrons trapped begins to degrade.
As damage accumulates, the voltage difference between “erased” and “programmed” states narrows. This makes it impossible for the SSD controller to reliably distinguish between 1s and 0s in that page or block. This is, in essence, a bad sector in SSD.
Why Bad Sectors Develop in Hard Drives: Main Causes
Bad sectors are an expected part of the lifecycle of every hard disk drive and SSD. Even during manufacturing, it is virtually impossible to produce a storage medium with millions of sectors or blocks and find zero defects.
This is why a bad sector in a hard disk or SSD may already exist before you start using the device.
Such flaws are called primary defects. Others appear later due to wear, accidents, or operational issues and are called grown defects.
1. Primary Defects (Manufacturing Flaws)
Primary defects are those bad sectors identified at the factory before the storage device reaches you. Manufacturing flaws tend to leave behind microscopic imperfections in the hard disk’s magnetic coating or the silicon structure of SSD flash cells.
In every manufacturing unit, there are variations in component assembly (subtle misalignments or tolerances during the construction of heads, platters, or NAND dies) which can introduce bad sectors in the storage media.
2. Grown Defects (That Develop Over Time)
Grown defects occur after you start using the device, and they keep increasing with usage. They can be categorized as physical (hard) bad sectors and logical (soft) bad sectors.
Types of Bad Sectors
1. Physical (Hard) Bad Sectors
These are caused by physical changes or damage to the storage medium. Some common causes are as follows.
- The read/write head may physically strike the spinning platter due to external shock or sudden movement. This is called a “head crash,” and it instantly creates one or more bad sectors.
- Dust particles or internal debris tend to become trapped between the head and disk and go on to scratch the platter surface.
- Mechanical wear causes gradual deterioration in a sector’s ability to hold data.
- When an HDD gets exposed to extreme temperatures (either heat or cold), its components can expand or contract and cause head misalignment, which leads to its unintentional contact with the platter.
- In solid-state drives, frequent program/erase cycles wear out the tunnel oxide layer in NAND cells and cause permanent loss of storage capability in affected blocks.
2. Logical (Soft) Bad Sectors
These are typically the result of logical issues that affect how data is written or maintained on the disk.
- If power is suddenly lost during a write operation, a sector may not complete its update and cause data corruption that can’t be repaired by Error Correction Code (ECC).
- In an HDD, vibration or minor shock can cause the head to write at an improper height or position (called weak writes or off-track), because of which data cannot be read back reliably.
- Firmware bugs or controller glitches can cause sectors to be incorrectly marked as bad or inaccessible.
How Are Bad Sectors Detected?
Detection of bad sectors takes place at two critical stages.
- First, during the manufacturing process (called factory screening).
- Second, during real-world use.
Factory Screening for Bad Sectors
Storage media manufacturers use highly sensitive scanning systems to test the entire storage surface of every new hard disk or SSD before they ship it for use. Any sector that fails to store and retrieve data reliably is classified as a primary defect.
Many manufacturers grade their drives by quality (e.g., A/B/C), based on the number of primary defects found in them.
Two main factory-level techniques are used to manage these sectors.
- Sector Slipping: If a bad sector is found, all logical sectors after it on that track are shifted forward by one physical slot. This is done to make sure that sequential data performance remains consistent and the defective area is never used.
- Sector Reallocation: If there are more defects than spare sectors in a single track, the controller redirects the logical address of the bad sector to a spare sector located elsewhere on the disk.
This entire process is transparent to the user. That is, when a drive is new, these mapped-out flaws are not visible to the operating system or via any standard tools.
However, it is possible for users to check for grown defects that appear during real-world use.
How to Check HDDs/SSDs for Bad Sectors
Once a drive is in use, bad sector detection becomes an ongoing process. There are three main methods for detecting and tracking bad sectors.
1. Operating System Diagnostic Utilities
- In Windows, you can scan the hard drive for bad sectors using the CHKDSK utility. It checks both file system integrity and, when run with the right flags, physically scans for unreadable sectors.
- On macOS, Disk Utility performs similar surface scans and error checks.
2. S.M.A.R.T. Monitoring
- Modern drives implement S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology), which records detailed internal health data.
- You can use third-party software (like CrystalDiskInfo) to read key SMART attributes like the following
- Reallocated Sector Count (Attribute 5): Shows sectors already swapped for spares.
- Current Pending Sector Count (Attribute 197): Sectors waiting to be retested or remapped.
- Offline Scan Uncorrectable Count (Attribute 198): Sectors that could not be recovered during self-tests.
- If any of these counts increase regularly, you should back up your data, as the drive may stop working properly very soon.
3. Observable Symptoms and System Errors
The presence of bad sectors could be indicated by some observable symptoms of hard drive failure.
- Unusual noises, such as repetitive clicking (also called the "click of death").
- Specific error messages such as:
Can Data Be Recovered From a Drive with Bad Sectors?
Yes, data can be recovered from a drive with bad sectors. However, your chances depend on the severity of the problem, the specific storage technology involved, and what you do next once you find the problem.
Data Recovery From HDDs
If you have a traditional hard drive with bad sectors, data recovery is possible in most cases.
- When a hard drive bad sector develops, it affects only a small, isolated part of the disk surface. The vast majority of user data is still fully accessible.
- Modern hard disks use advanced firmware and error correction to mask problems and allow data recovery utilities to read healthy sectors while skipping or retrying problematic ones.
- Professional HDD Data Recovery Services can use specialized hardware imaging tools that read the disk at a very low level.
- These tools can clone all readable areas, create a sector-by-sector image, and attempt multiple controlled retries around bad sectors to extract the maximum amount of data.
- In cases where files span both good and bad sectors, recovery specialists can reconstruct file fragments and metadata.
- If the drive is part of a RAID array, redundancy allows for missing data to be rebuilt using information from other disks.
Important: Repeated attempts to run consumer repair tools (such as CHKDSK or generic “hard drive bad sector fix” utilities) can make the situation worse. Aggressive scanning can accelerate mechanical wear or cause logical corruption and decrease recovery chances.
If you see symptoms such as a growing Reallocated Sector Count, repeated write errors, or the inability to copy files, stop using the drive and contact Stellar Data Recovery Service for the best chance of success.
Data Recovery From SSDs
Recovering data from bad sectors on an SSD (technically, bad blocks) is a bit more complex but still doable.
Professional SSD Data Recovery experts can safely remove the NAND flash chips and read them with specialized hardware. Then, they can manually reconstruct data with their advanced knowledge of file systems and the drive’s Flash Translation Layer (FTL).
Note: With SSDs, time and action matter even more than with HDDs. If you notice file loss, disappearing partitions, SMART warnings, or other symptoms of SSD failure, back up immediately and consult SSD data recovery services.
When to Seek Professional Help
You should consider data recovery services if:
- Your drive reports growing numbers of bad sectors (noticeable via S.M.A.R.T. statistics).
- Your files are disappearing or becoming corrupted.
- Your drive has become read-only.
- You hear unusual noises (for HDDs).
If your system reports bad sectors, treat it as a warning.
- Back up immediately.
- Avoid aggressive repair tools.
- Do not keep writing to the drive.
- Seek professional assessment if data matters.
About The Author
Somdatta is a professional content writer and analyst focused on the storage technology sector, with expertise in both magnetic and flash storage, as well as cloud computing and virtualization concepts. Somdatta translates technical concepts into clear, engaging content to sensitize readers toward a multitude of data loss scenarios and help them gain insights into the nuances of data recovery.
