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 Bad sectors (disk read errors) and what to do about it
Summary:
Disks can develop bad sectors over time, or by experiencing improper handling. Diagnose a disk using S.M.A.R.T. or by running a surface scan. Repair bad sectors in-place (if there are few) or clone a disk (if there are many).
Disk Read Errors

Data can be lost due to unreadable or 'bad' sectors. If a file occupies a sector that returns an error on read, the file will become unusable. If there are too many bad sectors you may even be unable to access an entire volume.

If the MBR or a sector that contains a partition table or boot sector can not be read, the entire disk or drive will become inaccessible. Bad sectors in a system area are a recipe for disaster.

There are 2 ways of dealing with data loss due to read errors:

1. Clone the disk to a "known to be good" disk
2. Repair bad sectors in-place

If you have reason to believe a disk's condition is rapidly deteriorating, always clone the disk!

If a partition was lost because the partition table sector could not be read, apart from repairing the bad sector you will also need to rebuild the information that was in that sector.

 
Cloning a bad or corrupt hard disk
To be able to clone a disk with read errors or logical corruption you will need a data recovery utility that is designed to cope with this type of error. The tool of choice must gracefully handle read errors and get as much data from the source disk as possible. It should ignore any (corrupt) logical structures on the disk. 

A successful strategy to copy as much data of a disk as possible is doing multiple passes. In a first pass try to avoid disk hangups by not trying too hard and skip over errors. The cloning software keeps track of the areas it skipped and processes those areas during the next pass. DiskPatch clones disk to disk using a 2 pass clone. iRecover writes a raw image file that can later be processed, also using a 2 pass clone.
 
Repairing corrupt or bad sectors
If a sector's data is corrupt, it can often be reconstructed using ECC (error correction). If the sector is too unreliable to store data again, it will be reallocated; the sector is taken out of service and replaced by one from the 'spare pool'. The spare pool is a group or multiple groups of spare sectors available on any modern hard disk.
You can detect unreliable sectors by scanning your disk's surface with DiskPatch. You can check the state of the disk by running a disk health check (S.M.A.R.T. check). iRecover also can check a disk´s SMART status.

If the data from a sector can not be read even after error correction, it becomes pending for reallocation; it is not yet automatically swapped.

On write a sector is reallocated if any of the 2 following events occur:
- a sector can not be written to, or
- the sector written to is pending reallocation.

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