Ever since personal computers were invented, I believe the most common problem all users have faced is 'Hard Disk Crash'. I thought of compiling the top reasons why a hard disk may fail. Here are few that came to my mind -
1. Physical Damage: When you carry your HDD with you, the R/W arm may get dislocated.
2. File System Corruption: Not sure why this happens though.
3. Malware / Virus related damage.
Some of the above reasons of failure can be avoided by using Solid State Drives (SSD). They do not generally suffer due to physical shocks. However, there is a catch and that is they cost much more than normal hard drives.