How common are bad blocks?
So common that every disk manufactured comes out of the factory with a list of bad blocks that were found during quality control. It would be highly unusual for a newly manufactered disk to have no bad blocks, however, most if not all of them should be caught by the manufacturer and will not affect your data. Bad blocks that develop later on in the life of the disk can be more common in units that have been mistreated (e.g. exposed to high temperatures, or placed in other environments likely to have an adverse affect on sensitive electronic equipment). But they can happen to any disk, at any time. Often you won’t notice them because of the sparing mechanism, and because modern disks typically use error correcting codes, so that a certain rate of read errors are tolerable. Indeed, some models now read and re-write data when they are not otherwise busy in an effort to clean up any errors that they do detect.
