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How to Fix Bad Sectors on a Hard Drive

The truth of the matter is; you can’t repair bad sectors on a hard drive. Receiving ‘Bad Read’ errors or I/O errors from a hard drive is not the type of problem that can be remedied or ignored. The following is an explanation of how hard drives are designed and how a bad sector revealing itself on your system is a slippery slope to disaster.

First of all, virtually every hard drive manufactured has bad sectors on it before it leaves the factory. A utility is run on the hard drive to find all the initial bad sectors and placed in a list. Every sector in the list is then remapped and saved to an area called the ‘Permanent Defect List’ or PList. It would be far too expensive to manufacturer a hard drive that every sector was perfect. The manufacturers take that into consideration so every drive that is made has a pool of sectors that is used only if a bad sector is found.

This process for pointing a bad sector to a new sector is called remapping and has been used for years. Internally the hard drive firmware will take a sector that is bad and point it to another sector. The new sector is now used in place of the old one, this process is transparent to the operating system and user. The sector is placed in a list like the one created at the factory; however this list is being created at runtime and is called a ‘Growth Defect List’ or GList because it grows with the life of the drive.

Now that we understand how a PList and a GList work let’s take a look and see why a single bad sector on a drive is an extremely bad situation to ignore. Since we know that all bad sectors are remapped to a pool of unused sectors, and the size of this pool is substantial, the only way a bad sector will show up on your system is if the pool has been completely used. In other words, your remapping pool is so full that it cannot take another bad sector and now the drive is throwing errors that it can’t read from a sector.

This is a clear indication that the drive is in a very critical state, has been for quite awhile and is deteriorating at an alarming rate.

In order to save your data it is now time to copy all of your data onto another hard drive. It is just a matter of time before a bad sector develops in an area that is very sensitive and the drive becomes inaccessible. There are many areas on the drive that house runtime critical data and if destroyed will make your hard drive a paper weight.

All of this being said there is one type of I/O error that may be overcome and that is a CRC error. Each sector has a validation code. This validation code is calculated and saved in an area right next to the actual sector data. This is called a long sector area, or a long sector read. When data is read from a sector the CRC validation logic is executed on the recently read sector and compared with the saved validation code. If the two codes do not match then a CRC error is thrown and a request for a new read is made. Several requests are made and if the CRC error is not resolved then a timeout is issued by the firmware and the sector is ignored. With that being said, what if the CRC code saved to the sector is bad or unreadable and the actual data itself is fine. What if a portion of the sector is fine but one byte is out of sequence. That would cause a CRC error as well.

There are special tools, and software that can ignore these errors and read the sector anyway. The data can then be copied to an image and used to recover your data. I want to caveat this by saying this is not a sector repair, once a sector is physically damaged and remapped it cannot be repaired, however some CRC errors with the correct software and equipment can be overrode. DTI Data uses such equipment everyday to recover someone’s irreplaceable data.

In summary, it can seem as if it is possible to recover a bad sector, however as I have clearly shown such is not the case.


12 Responses to “How to Fix Bad Sectors on a Hard Drive”

  1. Bob March 30, 2011 3:48 pm #

    I had a drive that started doing this. I kept using it and lost my data

  2. garylong February 3, 2012 6:07 pm #

    I have in the last year or so, some WD 1TB drives that have bad sectors going by what Hard Disk Sentinel Pro v3.70 have told me, I did a total format and hard drive come back to 100% life with no bad sectors, and the drive that come back to 100% had been going for some time now, but I will not put valuable date on them,
    can you tell me if I will expect them to come back with bad sectors over time, i had believe yes.
    I live in china but not Chinese and the price of drives have gone up very much for 1 and 2 TB drives, have been hard to get now, and one of the 2 TB drive only last 8 months of using it , I had bad sectors did a formate many times and used more then 4 other programs to regenerate it, but it has gone down from 90% health to 44% health for a drive that’s only one year old its a total waste, when I think some of the old drives I have over 10 years ago are still working, it seems to be the bigger drives are the more easy they are to get bad sectors!!!!!.

    • Jacqui Best February 10, 2012 12:53 pm #

      Bad sectors are the drive losing magnetism, this problem can not be fixed and over time the area that is cause the problem will continue to grow. I wish I had a better answer for you. I am hopeful drive prices will start to come back down now that WD has thier plant running again.

  3. Milan April 5, 2012 3:30 pm #


    I have external WD 320Gb that I would like to use for movies and music (nothing important) but the drive has 12Mb bad sector that I would like to remap, so I can use the drive almost as good as a working one 😀

    What tool or software would you suggest to use in my case ?

    THX and gretings from sunny Belgrade, Serbia, Europe

  4. Hardwarefreak May 17, 2012 10:35 am #

    I have a very old computer equipment with the ASUS P/I- P55T2P4 MoBo and was trying to install XP in it. It runs the setup(amost finalizes installation) and during middle of the setup, it goes to the Blue screen “BSOD”. It has 128 MB RAM and 200 Mhz Pentium processor.(No MMX). When I repeat the setup, the BSOD keeps occuring. Sometimes it shows ‘Driver IRQL Less or Equal’ error or other times just plain BSOD without any driver error specified.
    The problem being it would not boot from CD-ROM just because CMOS settings always get back to default bios settings to boot sequence master HD and floppy. I would like to force the CDrom boot so that I can carry out the harddisk proper format and install…just need help on how to do that…

    • Jacqui Best March 7, 2014 12:27 pm #

      If the BIOS will not say the boot configuration to at least have the CD-ROM drive as one of the ones that it can boot from I would say there is no work around without creating a windows boot floppy and going that route.

  5. Kerry March 4, 2013 3:39 pm #


    If the problem whit the sector isn’t really that it has magnetism but a CRC error, will it help to use a diagnostics software or format the disk or does that not matter?

    • David Mohyla March 6, 2013 4:03 pm #

      Yes, you can do a full format of the drive or there are third party tools. However, this will destroy your data.

      • Kerry March 10, 2013 5:11 pm #

        Yes I know that but if it will restore areas that aren’t really bad but suffering from CRC-errors I can move the data to a different hdd, format the one whit the problems and then move the data back to the original hdd. This is if there is a slight chans of improving the drives health, otherwise there is no point in trying.

  6. Hatem April 21, 2013 6:05 am #

    First of all thank you for the excellent article.

    Secondly I have a problem and hope some one could help me with.

    I have a hard disk with bad sectors preventing me from accessing it, and I want a tool/software could be run bootable without OS that can read the HD regardless it’s bad sectors and/or copy all good sectors to a new HD and regenerate the data if applicable.

    Hope any pro can help me


    • David Mohyla April 22, 2013 11:02 am #

      Hi, try cloning your drive in DOS or using something like DD. Then work on the good clone hard drive.

  7. Donn Edwards May 22, 2013 2:13 pm #

    I’m not sure what recovery tools your article is referring to, but the most obvious one to begin with is Spinrite, which will read the damaged sector and attempt to retrieve as much data as possible, before writing this data to an undamaged sector and requesting that the drive controller mark the problem sector as bad.

    It also has a maintenance mode which reads each sector of the drive and re-writes it, thereby refreshing the magnetic signal and avoiding the main cause of lost data in the first place.

    @Hatem: I would try Spinrite first, if you haven’t already done so.

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