This is what a disk crash looks like...

If you've ever wondered what goes on inside your hard disk, now's the time to find out.  I recently had to try and recover some data from a disk, but with no success.  I decided it might be useful to see what had happened, so I opened the disk up - by the way, these pictures are quite high res, so they may take some time to load, but it's worth it...

A bit of explanation is called for here.  Your data is magnetically stored on the platter (the large silver disk) by the read write head, which is moved over the surface of the disk by the arm.  In the next two pictures, you can see the circular scratch over the disk, caused when the head smashed into the platter...

End result?  Damaged head, damaged surface, loss of data.  Moral of the story?  ALWAYS back up your data to an extral hard drive, or dvd.