Hard Drive Magnets

I would like to know what is the purpose of the very strong magnets in a hard drive?

And why do they have to be so strong as compared to a magnet for a recording head of a tape recorder?

Also, I would like to know, how is it possible that one side of the magnet is so extremely strong, and on the other side of the metal plate which it is attached to, there is no noticeable magnetism? What is the material used on that side of the magnet?