Bad sector
A bad sector is a small block on a disk that the device can no longer read or write reliably.
Disks store data in numbered blocks called sectors. A sector goes bad when the surface is weak, when the heads struggle to follow the track, or when the drive has run out of spare blocks to substitute. The drive often hides a failing sector by moving its contents to a spare. That is ordinary maintenance until it starts happening often.
How do bad sectors show up?
Reads become slow, a copy fails part way through a file, or the drive disappears when it is asked to work hard. SMART attributes can report sectors the drive has already replaced, and sectors it is waiting to replace. A count that keeps rising is a warning. It is not a date on which the disk will fail.
Why do bad sectors hide files?
A file can be read only when every sector it uses can be read. One unreadable sector can break a document, a database page, or a record in the file system. If the bad sector sits inside the file system's own index, the whole volume can look empty or raw.
What should you do if a disk is developing bad sectors?
Copy the files you still need onto another device, and stop treating the failing disk as the only copy. A surface scan that writes to the disk, or a program that retries one sector for a long time, can push a weak drive further. Persistent abnormal clicking can indicate a physical failure. Seagate says to confirm a backup straight away, and that a grinding sound means continued use can damage the platters, so stop using the disk and leave it powered off.
What might you notice?
- Copies of large files fail at the same point each time
- The computer hangs when that disk is opened
- SMART reports reallocated or pending sectors
- The drive is slower or noisier than it used to be