The most important thing to remember about hard disks is that they will fail. Often, they will do it without warning. So you must make back ups and keep them current.
Recently, I had a drive fail with excessive bad sectors. It happened after I had moved a new set of photos off my memory card, but before I had copied the files to my backup drive. The bad sectors corrupted the directory structure on the drive, but I doubted the actual file data was effected. It was time for a PhotoRec recovery.