Bad block or faulty sector is the name given to a damaged area on a hard disk. It is a physical problem, i. e., the hard disk’s magnetic media is defective. When we run a disk utility such as Scandisk and Norton Disk Doctor, such faulty sectors are marked with a "B".
Several users have written us asking how to proceed to recover hard disks with bad blocks. Many note that bad blocks disappear after low level formatting the hard disks.
What really happens, however, is that current physical formatting programs do not actually physically format the disk. If this should be feasible, the hard disk would be damaged, since hard disk tracks have a signal called servo that operates as a guide for the hard disk head. If we really formatted a hard disk at low level, these servos would be erased and the hard disk head would be unable to move any longer.
Low level formatting programs are utilities for detecting bad sectors and wiping the disk (for security reasons, for instance, after concluding a confidential project), not carrying out – despite their name – low level formatting.
These programs have an interesting function, which consists of updating the disk’s bad sector map. When you use this option, the program scans the disk, seeking defective sectors and updating the disk’s map.
When you run a high level formatting (through the Format command), this command skips the sectors contained in this bad sector table. According, there will not be any sector marked B ("Bad Block") in the FAT, although the defective sectors remain on the disk.
Defective sectors are not removed, but merely noted in this table of bad sectors, resulting in the system ignoring them (in other words, the sectors are hidden).
If new bad sectors keep occurring after running this procedure, you should get rid of the disk, as its magnetic surface is deteriorating, for some reason.
The best program to be applied in the procedure is the manufacturer’s one, given on the respective utility page.