A disk checking tool has reported bad sectors, or Windows has started to hang whenever you open a particular folder. It is tempting to run a repair straight away, but that is often the wrong first move. Bad sectors are a sign that a hard drive is having trouble reading its own data, and if the files on it are not backed up, the priority is to get them off rather than to fix the drive. This guide explains what bad sectors are, what the warning signs mean and which “repairs” can make things worse.
What a bad sector actually is
A hard drive stores data in small blocks called sectors, usually 512 bytes or 4,096 bytes each. A bad sector is one the drive can no longer read reliably. There are two broad kinds:
- Soft bad sectors are logical errors. The data and its error-correction information no longer agree, often because power was lost during a write. The surface itself may be fine.
- Hard bad sectors are physical. The magnetic surface may be damaged or degraded, or the read/write heads may be getting too weak to read that area.
From the outside the two look the same, which is why a scan result alone does not tell you how serious the problem is.
Signs your drive has bad sectors
- Windows or macOS freezes for several seconds when opening certain files or folders
- File copies stall at the same point, or fail with a read error
- Some files will not open, or open damaged, while others are fine
- The computer has become very slow to start for no obvious reason
- SMART tools show Reallocated Sectors, Current Pending Sectors or Offline Uncorrectable counts above zero
The two SMART counts worth understanding are reallocated and pending sectors. A reallocated sector has already been retired and swapped for one from the drive’s spare area. A pending sector is one the drive could not read and is waiting to deal with. A small, stable reallocated count may stay that way for a long time. A count that is rising, or any pending sectors alongside the symptoms above, suggests the drive is actively getting worse.
Why a few bad sectors can be a bigger problem than they sound
Bad sectors are rarely spread evenly. They can be a symptom of something else: a head that is weakening, debris on the platter after a knock, or a small area of surface damage. When a drive meets a sector it cannot read, it retries again and again. Those repeated attempts add wear to a component that may already be failing. A drive with a handful of unreadable sectors today may have thousands next week, and in some cases a weak head fails completely and the drive starts clicking.
Location matters too. If the bad area falls in the file system’s own records, a healthy-looking drive can suddenly show as RAW, ask to be formatted or lose whole folders, even though most of the data is still there.
Why “repair” tools can be risky
This is the main reason to pause before doing anything.
- CHKDSK /r reads the whole volume, which can take many hours on a large drive. That is a heavy workload for a drive that is struggling. It also changes the file system as it goes. It marks bad areas so they are not used again, but it does not bring back data that cannot be read, and files it cannot make sense of may be moved or removed.
- Bad sector repair or regenerator utilities work by repeatedly reading and writing to the problem areas. Any write to a sector replaces what was there, so the contents of that sector are gone for good.
- Low-level or zero-fill tools make a drive look healthier by forcing it to reallocate sectors, but they do so by erasing data.
- Ordinary cloning software may give up at the first read error, or may spend hours retrying and put more strain on the drive.
All of these can be reasonable on a drive whose data you no longer need. They are the wrong tools when the data is the point. A drive that has developed bad sectors should not be trusted again anyway, even if a repair tool reports success.
What to do instead
- Stop using the drive for everyday work. Every hour of use is more wear, and a system drive is reading and writing all the time in the background.
- Do not run CHKDSK, disk repair or a defragmenter on it. If Windows offers to scan and fix the drive at startup, decline it if you can.
- If the drive is still working well and only a few files matter, copy those first. Start with the most important folders and copy to a separate, healthy drive. Avoid copying everything in one large batch.
- Stop if it gets worse. If copying stalls, the drive starts clicking or it disappears from the system, power it down rather than trying again.
- Write down what happened. Note any error messages, what tools you have already run and when the problems began. It helps with diagnosis.
How bad sector recovery works
The professional approach is to take a sector-by-sector copy of the drive, known as an image, using equipment designed for unstable disks. Unlike normal software, it can limit retries, skip difficult areas and come back to them later, so the easy data is secured first and the drive is put under as little stress as possible. The files are then rebuilt from the image, not from the original disk. If a head has weakened, the drive may first need work in a clean room. You can read more on our hard drive data recovery and hard drive repair pages, and if the drive is inside a notebook, our laptop data recovery page explains the options.
Talk to Data Doctor
If the drive holds files you cannot replace, stop running scans and call Data Doctor on 020 7394 2529 for a free initial consultation, or bring the drive to us at Unit 1 The Willows, 80 Willow Walk, London SE1 5SY.