Fans and transformers which are inside the server case create magnetic and electric fields.

Electric fields can be easily shielded, but what about magnetic fields, they can only be shielded with high cost materials like mu metal http://en.wikipedia.org/wiki/Mu-metal

If a hard drive is installed too close to an intense transformer field, how is the magnetically stored information on the ferromagnetic surfaces of the disk kept safe? Even if drives are shielded, where are the limits?

Is there some technical investigation or recommendation from manufacturers about this?

(I never heard about something and never had any problem but I am interested in some facts. This is much preferred over what you believe or a habit you developed. Please try to give some solid infromation.)

I have built and repaired many servers and sometimes I did put the harddrive on top of the power supply.

Edit: This question is not about frequencies that could affect the drive via the power or data connectors of the drive, those are electronically decoupled and that's another question.

Edit 2: The wikipedia page states that the motor inside the drive is shielded with mu metal. It is obvious that manufactureres have to take care of this. This question is about such influences from outside the drive.