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Concept
Bad sector
Summary
A bad sector in computing is a disk sector on a disk storage unit that is unreadable. Upon taking damage, all information stored on that sector is lost. When a bad sector is found and marked, the operating system like Windows or Linux will skip it in the future. Bad sectors are a threat to information security in the sense of data remanence.
Bad sectors can be "soft" (logical) or "hard" (hardware, physical), depending on what is making the sector inaccessible. In case of power loss, bit rot (more likely on floppy disks), or firmware issues, the on-disk format can be corrupt beyond what the error correcting code can fix. This is a "soft" bad sector: writing over the corruption would succeed.
On the other hand, sectors broken physically cannot be restored: writing would fail, forcing a remap. A new drive may start with some innocuous bad sectors due to manufacturing flaws. Larger patches occur throughout use, due to head crash, wear-and-tear, physical shock, or dust intrusion.
Bad sectors may be detected by the operating system or the disk controller. Most contain provisions for sectors to be marked as bad, so that the operating system avoids them in the future. Disk diagnostic utilities, such as CHKDSK (Microsoft Windows), Disk Utility (on macOS), or badblocks (on Linux) can actively look for bad sectors upon user request.
With the advent of SMART-enabled disk controllers (see below), the burden of avoiding bad sectors more commonly falls to the disk. Some newer file systems such as Btrfs and ZFS do not have a bad-block avoidance feature at all. Software tools that look for bad blocks still have a use case: by issuing writes at detected bad sectors, one can expedite the remapping process, avoiding further attempts at reading the bad sector.
When a sector is found to be bad or unstable by the firmware of a disk controller, a modern (post-1990) disk controller remaps the logical sector to a different physical sector. Typically, automatic remapping of sectors only happens when a sector is written to – failed reads remain marked "pending".
Official source
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