What are bad sectors?
Hard drive media stores data in sectors or magnetic domains.
Each sector is typically 512 Bytes although this can vary depending of the file system.
Bad sectors are areas of the disk which are not accessible, certainly by ordinary means however some bad sectors can be read using special hardware and knowledge.
A file system will try and ‘mark' the bad sectors in the $meta data $bad, this will flag the operating system not to use sectors with this attribute, however the hard drive also has a bad sector handling capability, this is broken down to the P-List or factory list and is compiled and generated at the factory utilising so called internal formatting, this list is vital to the hard drive functioning and will remain with the drive until its end of life and helps to make the translator of the hard drive, the translator is a complex firmware module which maps the physical to logical geometry of the hard drive, further to the P-List there is the G-List or growing defect list, this area literally grows as the drive ages and is used, these bad sectors are then replaced with remapping of new ‘reserve' good sectors, of course this has a finite limit and can become ‘full' or have no reserve for bad sectors it can also become damaged or fail, this will halt the hard drive and stop normal operation of the hard disk drive, this is a relatively common occurrence and failure type.
What causes bad sectors?
Causes of bad sectors can be through heat, the hard drive becoming very hot which they frequently do, this excess heat is not a good mix for the magnetic properties of the disk surface, additionally to this hard drives wear out magnetic domains fail, they ‘loose' properties and have a shorter than anticipated start stop cycle or mean time between failure, particular areas typically ‘wear out' or fail before other sectors this is due in part to the operation requests of the file system and the need to read repeated areas of the surface, in a Windows based machine running NTFS file system the MFT master file table will typically start at 12.5% or 6,290,000 LBA this area contains the information for all files that reside on the hard drive and is accessed very frequently.
Some clients attempt to ‘fix' the bad sectors this is not a good idea, as firstly the software will not deal with the bad sectors at a level that is required as the information relating to bad sectors is dealt with using firmware and ATA command set, secondly the software can make bad sectors when there are not, and thirdly it may cause serious issues for any data recovery attempt or existing data at a later point.
A good indicator that your hard drive has bad sectors can be reported from the SMART modules which will inform of reallocated bad sector count and the current threshold it is currently at, if you have a hard drive which has reported SMART failures or issues and it is under warranty backup you important data immediately and RMA the disk back to the supplier for replacement, make sure that you motherboard and BIOS have SMART enabled many in fact do not, if you are running other operating systems it is possible to download software from the internet that can report in live time the status of your SMART.