![]() In IDE drives this all happens automatically, without the need for operating system intervention. ![]() SCSI drives have a command, IOCTL_DISK_REASSIGN_BLOCKS, to tell them to reallocate a bad spot on the drive after the operating system detects it. If you performed a low-level format of the drive, you had to use a tool to type in all the Cylinder-Head-Sector locations of the bad spots. This sticker contained the Factory Defect List the list of all known bad spots on the drive. The drive itself can handle those details itself.īonus Chatter: In the olden days, your hard drive shipped with a sticker fastened to it. It means the host operating system doesn't have to deal with the issue of failing sectors. These actions of re-reading sectors, consuming spares, all behind the computer's back is a good thing. This means that there were four occasions where the drive marked sectors as bad, and used spare sectors instead. On this same hard drive, there have been 4 Reallocation Events. That means that 64 of the drive's spare sectors have been called into use: ID Current Worst Threshold Raw In this example from one of my own hard drives, 64 sectors were found to be bad. And the total number of sectors that have been reallocated (and so how many of your spare sectors have been used up) is the Reallocated Sector Count. This marking off of a bad sector, and reallocating its data to a spare sector, is called a Reallocation Event. 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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