What are bad sectors – MJM Data Recovery Ltd

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What are bad sectors?

What are, and what causes bad sectors?

This is the question we are most often asked, as it is one of the biggest problems we see with hard disk drives.

Bad sectors: overview

One of the main reasons people look to data recovery is because of bad sectors. The user might not realise that the problem they have, is related to bad sectors. The reason is, that the symptoms can be very misleading, especially if you have never experienced it before.

Symptoms of bad sectors can show different symptoms. Some are.

When bad sectors start developing on a hard drive, when you try to access a file, or folder on the drive, the operating system will attempt to read the bad sector. You may notice that it's taking a long time to open the file. The computer continues to attempt reading the file, until either the hard drive returns an error, or can even cause the computer to crash.

If there are bad sectors located in files that are needed to start the computer, the same symptoms as above may occur. Or, for example, Windows may give a message saying Windows needs to be repaired.

It could also be that the computer will not start at all, and freezes at the power up/POST screen.

When bad sectors are detected, the operating system may attempt to repair them. Windows uses a program called 'Check Disk' or chkdsk to attempt to repair bad sectors.

Running recovery software like this can give you the same results as the 'Slow access' section above, and takes an inordinate time to run. Very often just freezing the computer. The same goes for most data recovery software that is simply not designed to work with bad sector problems.
If you have dropped your hard drive, computer, and you get similar results to those above. Then we recommend that you turn it off, especially if it starts clicking or ticking when trying to read or write data.

This is indicative of a head crash, which can be exacerbated when turning on the computer, or plugging in an external drive, after it's suffered physical shock or damage.

Firstly, there are two types of bad sectors, these are known in the data recovery trade as 'soft' or 'hard'.

Soft bad sectors
A soft, bad sector is usually associated with a bad write to a file, for example when saving the file after an edit. This is common with external hard drives, since the data is stored in buffer-memory before writing to the drive. If the drive is unplugged, before the buffer is completely written to disk, then bad sectors can appear the next time you try to open it. So, always use the eject option when removing an external hard drive or memory stick from your computer, as this will flush the buffer and save to disk.

Soft bad sectors are not typically detrimental to hard disk drives. If the drive is SMART enabled, then the drive management system will 'swap' the bad sector for a new one from the 'spare sector pool' (We go into this in more detail in another article on bad sector remapping).

However, the file may now be corrupt, as at least one sector will be missing from the file and replaced with an empty one.

When the spare sector pool is running out of replacement sectors, you may get a message on Windows suggesting that you back up and replace the drive. This warning should be heeded with serious, and prompt consideration. When the bad sector pool is used up, the drive is unable to replace any more bad sectors. At this point, you will start seeing symptoms, like those described above.

Hard bad sectors.
These have similar symptoms as soft bad sectors, and likely to cause some ticking noise and are not repairable with chkdsk or any other software. The data once stored in these bad sectors is lost.

They are usually caused by disruptions in the magnetic flux caused by failure of the surface of the platters inside the hard drive. The platter surface made of magnetic particles, when one fails it leads to the bad sector. If you consider, a 1 TB drive contains a billion bytes and there are 8 binary bits in one byte. Then the enormity of what is happening can be understood.

Data is not actually stored ON the platter, it is stored in the magnetic flux, just 0.3 nano metres above the surface of the platter, each binary bit can be a 0 or 1. A tiny coil in the write head will either be charged as +ve or -ve, this makes the flux pattern alter shape. When data is read, the read head coil is passed through the flux and, depending on the resistance returned, will be converted back into a 1 or 0.

When a hard bad sector is found, the drive will report that it cannot determine the value correctly by sending an unrecoverable (UNC) error back to the computer.

How can we help?

MJM Data Recovery Ltd. has been in business since 1999, over this time we have developed technical skills and advanced techniques for recovering data from hard drives suffering with bad sectors.

We use the latest technology to make a clone of your hard drive, primarily skipping areas with bad sectors, without the errors or symptoms seen when accessing the hard drive normally on a computer. The software linked to the imaging hardware can let us, image by individual heads, image forwards and backwards, selectively image areas only where data is stored, and even select individual folder or files.

After a primary pass, we then read the drive backwards, to fill in the gaps that were skipped during the primary pass.

Two or more, further passes are made with increasing attempts at retrying to over read the bad sectors, sometimes even by modifications to the firmware and bad sector maps stored in the system area of the drive.

For a typical drive with bad sectors, firstly, we will only clone the areas on the drive where your critical data is stored. We then read the remaining data areas and finally complete the clone of the hard drive by reading all the remaining sectors.

During these phases, we write a text signature to the clone on the sectors that are unable to be read from your drive, this effectively identifies the files containing bad sectors. When we generate a list of the data on your drive, we can scan each of the file’s content, and when we find our marker text, we know it contains bad sectors. Our file list shows you which files are affected, so you know the scope of the problem and which specific files are specifically affected.

We wrote the file listing software early on, as we understand the importance of our customers being sure that we have recovered their data. In 2007, we decided to market it to other data recovery companies. Since then, it is being used by hundreds of data recovery companies throughout the world.

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