Bad sectors can occur in many different ways, and all drives will have bad sectors at some point in their lives. Modern hard drives typically use less aggressive "sector sparing" (more on this later) than older drives, so you will usually see more bad sectors than on older drives.
Hard drives will try to automatically "map out", or relocate, bad space. This is done completely transparent to the user, so you'll never know it's happening or that bad space is present. When a bad sector is "mapped out", it's data is moved to a "spare". Each region of the drive has a predefined number of spares, referred to as it's "sector sparing scheme". If there is a small concentration of bad space, all of the spares for a given area may be used up. When this happens, the drive is unable to relocate the bad sector and you then see the bad spot marked in SCANDISK.
Hard drive media (the actual "discs" inside the drive) is never perfect. There are always small imperfections that will contribute to bad space. Some hard drives are lucky enough to have this bad space evenly distributed around the media, so that the sector sparing scheme of the drive can adequately relocate them, and you never see bad space in SCANDISK. Some drives, particularly newer, higher capacity drives, use a limited sector sparing scheme (since lowering sparing increases capacity) and therefore you will "see" more bad space.
There are occasions where an improper write, virus, or errant program can accidentally mark a sector on a hard drive as "bad", when in fact no physical defect is present. These types of bad sectors can be repaired with some programs (Norton Disk Doctor, etc.), or a low-level format. A low-level format will remove all data from the drive. Bad Sectors can also be reported if you have noise in the data from faulty cables or other components. These "bad sectors" will show up on one scan, but move or change on the following. Low-Level formatting will not fix this, but you can try new cables or controllers to troubleshoot this occurrence.
Seagate ATA drives can be Low-Level formatted using Disk Manager which is included with our DiscWizard download. BIOS formatters and other software are not recommended for formatting Seagate IDE drives.
Seagate SCSI drives can be formatted using the Format utility that is provided by the SCSI controller maker. These utilities are usually in the BIOS of the card or come on the diskettes or CD-ROM's that come with the controllers. Please contact the controller maker for more information.
Generally a drive is not considered bad with just a few bad sectors. If the number of bad sectors grows or the bad sectors occur in the boot track then the drive should be replaced.