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'.
sectors are also referred to as the 'grown defects list'.