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Disk Surface Scanning with DiskPatch

Summary:
This page was created to give you a brief overview of a specific DiskPatch feature.
Before continuing with a procedure please make sure you consult the manual.
Surface scan & repair
Surface 'defects' are a common cause for data loss; individual files, folders or entire disks and partitions can become inaccessible due to unreadable sectors. Standard tools included with the Windows operating systems (Chkdsk, Scandisk) only flag (not fix) unreadable sectors (bad sectors) in the file system, thus making sure the operating system will not read from or write to the affected clusters.

Chkdsk and specifically Scandisk can not do anything useful at all when file system specific structures (like the FAT) are affected by bad sectors. DiskPatch addresses sectors on a level below the file system. By detecting bad sectors at this level, DiskPatch allows the disk internal error management to reallocate bad sectors; modern hard disks are equipped with a pool of spare sectors which can take the place of a bad sector. The bad sector itself is added to a list of sectors that will never be used again (the so called defect list). In effect, on a partition where the FAT was crippled by one or more bad sectors and could not be repaired by Scandisk before, repairs will be possible after the bad sectors were reallocated.

Some areas can not be scanned by Chkdsk or Scandisk. An example of such a structure is the most important sector on your hard disk (as it is the starting point from which all disk structures on your disk branch); the Master Boot record (MBR).

If DiskPatch detects a bad sector it will be dealt with according to one of the following strategies:

  • Error on read that can be corrected: hard disks apply a number of strategies for correcting 'bad' data, allowing the disk to provide you with your data even if a sector could not be read entirely. If the data in a sector can be recovered using this strategy, the data is copied to a spare sector and the bad sector is taken out of service. In addition to the methods the hard disk applies for recovering data, DiskPatch can try more re-reads and disk resets before trying to read again and get a 'good read'.

  • Error on read that could not be corrected: if a sector can not be read and the disk's automatic error correction can not recover the data in the sector, the sector will become a candidate for reallocation. A read error will be reported, the data in this sector is lost. The candidate for reallocation will be reallocated when the sector is written to. It is this type of event that will show up in SMART monitoring tools as 'Pending_Reallocation'.

  • Error on write: the bad sector is taken out of service and the data is immediately written to a spare sector. In other words, the bad sector is 'reallocated'.
    Reallocated sectors are also referred to as the 'grown defects list'.

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