Any requests to read or write to that damaged sector will transparently be redirected to a spare sector. If the drive recognizes a sector as bad, it will stop using it. Most modern drives contain a number of "spare" sectors (e.g. It will keep retrying until it gets a good value, or it's reached it's time limit (formally known as the Command completion time limit, or CCTL).ĭuring these retries, the drive will appear dead as it is no longer responding to commands. The hope is that if it simply reads it again, it might get the correct sector contents. If the drive detects that the contents of the sector are invalid, it will retry the read. The hard drive knows if the value it read is valid or not, because the drive uses Error-correcting code (ECC) to validate that the contents it read are correct. Normally the drive reads the sector, returns it to the host machine, and everything is fine. The host computer simply asks the drive to return the contents of a particular sector number. Hard drives today try to hide bad sectors from the host computer. Short answer: Write something new to the sector (even zeros - which a long format does).
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