Zeroing a disk maps out bad sectors?

By rodion15
Aug 7, 2015
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  1. I don't understand why sectors that cannot be read are left there by the disk firmware instead of copying its data (even though it's bad, there're EEC code mismatches or whatever) to a spare sector and them mapping out the bad sector. Anyone can explain why?

    I guess zeroing a disk (with disk utility in macs for example) maps out bad sectors doesn't it?
  2. jobeard

    jobeard TS Ambassador Posts: 9,143   +595

    A bad sector (by definition) can't be written / reread reliably, so there's no point attempting to zero it. Instead, it is marked as a bad sector, put on the bad list, and another is mapped into the file that experienced the issue.

    bad sectors stay as bad sectors. Fresh from the factory, your HD can have them - - a new disk is not required to be flawless.

    The low-level format will update bad sectors, but not recover any as good.

    The CHKDSK c: /F /R will examine fix and 'remap' bad sectors
  3. Techpumpkin_WD

    Techpumpkin_WD TS Member

    Hello there,

    When a sector cannot be read the data cannot be restored, the HDD can only tell if it's correct but it cannot recreate the damaged data. That is the reason why the sector will be left as a pending one until the data is either read correctly or there is an attempt to overwrite the sector. Zeroing the drive will write a zero to each sector, hence there will be an attempt to overwrite all the pending sectors. If the write is successful the sector will be used and no longer marked as pending, if the write fails, the sector will be reallocated.

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