The hard disk's file structures have become corrupted or the data on it is unreadable (an existing disk volume is acting as if it is unformatted, or there are very serious disk errors)

Explanation: The hard disk is generating error messages or is otherwise behaving as if it has become seriously corrupted. An existing, working hard disk may be acting as if it had been wiped clean, or had been never formatted. Strange error messages or very large quantities of files may be corrupted or wiped out. Note that small numbers of lost clusters or other minor file system corruption are often a result of more benign situations and are discussed here instead. (Bear in mind that if you don't scan for file systems regularly, they can accumulate and make the situation look a lot more dire than it really would be if the disk had been maintained properly.)

Diagnosis: There are a myriad of possible causes for a hard disk that is experiencing a large amount of corruption; most of them are, unfortunately, pretty serious. In many cases it is not the hard disk itself but rather an external factor that is causing the problem.