If your hard drive has bad blocks, you have a problem. It means part of the storage surface is damaged and can no longer be read reliably, so you may lose access to the data stored in that area.
The good news is that the data is often not lost for good. In many cases, photos, documents and other files can be recovered from a drive with bad blocks, as long as you take the right approach and don’t try to fix the drive yourself.
Below, we explain what a bad block is, what causes it and what to do when it happens.
What is a bad block on a hard drive?
Bad blocks are areas of a disk that can no longer be read or written reliably. They usually appear when something damages part of the drive’s surface.
A damaged area like this can slow the computer down, stop it from booting and make the data stored there inaccessible. A block is made up of one or more sectors, and a damaged sector is called a bad sector. So a bad block contains bad sectors, but not the other way around. Despite the difference, the two terms are often used interchangeably.
To understand what a bad block is, it helps to know how a hard drive works. HDD stands for hard disk drive, named after its key component: the disks, or platters, where the data is stored.
When you save a document or photo on your computer, for example, the file is written to sectors on the platters, which work like small data “drawers”.
As the platters spin at high speed, the read/write head, mounted on a moving arm, writes data to the disk or reads it back and sends it to the computer.
A bad block means that one of these storage areas has been damaged and the data stored there can no longer be read.
What is the spare (reserved) area?
Modern hard drives set aside a pool of spare sectors, often called the reserved or spare area, so they can keep working when individual sectors fail.
When a sector goes bad, the drive’s firmware usually detects it and stops using it. From then on, any request for that address is redirected elsewhere, so the drive doesn’t keep retrying the damaged spot, which would slow down or freeze the computer.
In other words, physical bad sectors can’t be repaired, only marked as defective and taken out of use.
The damaged sector is then replaced by one of the spare sectors in the reserved area. If the drive can still read the original sector (sometimes only after several retries), it copies the data across; if it can’t, the data in that sector is lost and the spare sector is used for new writes.
This process of mapping the address of a damaged sector to a spare one is called remapping or reallocation. It can happen many times, entirely in the background, without you ever noticing.
However, the number of spare sectors is limited, and a growing number of reallocated sectors is a sign that the drive is deteriorating. When remapping becomes frequent, make sure your data is backed up and replace the drive.
What causes bad blocks on a hard drive?
Bad blocks can be physical (hardware) or logical (software), and they’re often caused by misuse or accidents.
A physical bad block is real damage to the platter surface, and it can have several causes. The drive may have been dropped or knocked while running, causing the head to touch the platter and scratch it. Or someone may have tried to open the drive outside a clean room, letting dust or moisture in.
Hard drives are sealed and well built, but they are mechanical devices with delicate, high-precision parts and are sensitive to shocks, especially while running. That makes them prone to mechanical failures that affect how they work.
A logical (soft) bad block, on the other hand, isn’t physical damage: the sector’s data no longer matches its error-correction code, usually because a write was interrupted. This can happen after a sudden power cut or when you unplug an external hard drive without ejecting it first. Viruses and malware don’t damage the platters, but they can corrupt the file system and cause errors that look similar.
How can you prevent bad blocks?
As you can see, bad blocks are often the result of improper use, so most cases can be avoided. Look after your drive with these tips:
- Handle and transport your devices with care, especially laptops and external drives, which are moved around more often;
- Keep devices on stable, secure surfaces where they can’t be dropped or knocked;
- Never open a hard drive at home: outside a clean room, dust from the air can get inside and damage the platters;
- Use antivirus software, only connect your external drive to trusted computers and be careful with downloads to avoid infections;
- Use a UPS with your computer, and unplug it during power fluctuations to avoid sudden shutdowns;
- Always shut down your computer properly (Start > Power > Shut down) and use “Safely Remove Hardware” or “Eject” before unplugging an external drive.
Even with all these precautions, you should always keep a backup of your files, for example on another external drive and in the cloud. Keeping your data in two different places is the best way to make sure you don’t lose it, even if the drive fails completely.
How do I know if my hard drive has bad blocks?
Your drive won’t always warn you when a bad block appears. Just as with a fever or other illness, you need to look at the symptoms to reach a diagnosis. Checking the drive’s S.M.A.R.T. data with a diagnostic tool can also help.
Common symptoms of bad blocks include:
- Slow or frozen boot when loading the operating system;
- Error messages when trying to repair the system;
- Error messages when opening or copying files from the drive;
- The computer struggles to recognize the drive (in the case of an external drive);
- A blue screen when you try to start the computer.
These symptoms appear when the drive can no longer read some sectors and hasn’t been able to reallocate them, or when the damage keeps spreading. This is more common on older or failing drives.
When the drive manages to reallocate a few bad sectors to the spare area, you may not notice anything at all and can keep using it normally.
However, when bad blocks keep appearing and the spare area starts to run out, more serious symptoms can follow.
How do I recover data from a hard drive with bad blocks?
If your files are stored in sectors the drive can no longer read, you won’t be able to access them normally. In that case, you need professional data recovery.
If the drive has already reallocated the damaged sectors, the rest of the data is still accessible, but whatever was lost in those sectors won’t come back on its own. And if bad blocks become frequent and affect how the drive works, you’ll also need data recovery to get your files back.
There are several things you can do to recover a hard drive with bad blocks. But keep in mind that none of them repairs the physically damaged surface: the goal is to recover the data.
First, as soon as you notice symptoms of bad blocks, stop using the drive. Every hour it keeps running can make the damage worse.
Don’t run repair tools such as chkdsk /r or surface scans on a drive that holds important data: they force the drive to read every sector over and over, which can make a failing drive deteriorate quickly. Data recovery software doesn’t repair sectors either; it only tries to copy the files that can still be read.
Repair tools such as CHKDSK find the problem areas, mark them as unusable and try to fix file system errors. With logical bad blocks, that may solve the problem, but only run them once your data is safe elsewhere. Even then, you need to know exactly what you’re doing, because a mistake can lead to permanent data loss.
That’s why the safest option is to contact a professional data recovery service. With logical bad blocks, the chances of recovering the data are much higher. With physical bad blocks, the drive needs specialized work, such as imaging it with hardware that handles unreadable sectors, which only experienced professionals can do safely.
How Bot can help
A specialized data recovery company like Bot is the best way to deal with bad blocks on a hard drive.
With more than two decades of experience, Bot handles a wide range of data loss cases, with qualified specialists who assess both physical and logical bad blocks and recover the stored data.
When a drive has to be opened, the work is done in our Class 100 (ISO 5) cleanroom, using specialized equipment that gives you the best chance of getting your data back.
That way, you have the best possible chance of accessing your files, documents and photos again, without the worry.
Contact us today for a free standard diagnosis. We can also collect your device free of charge (islands not included). Fill in the form and start recovering the data from your hard drive.
