It sounds like your harddrive may have a bad magnetic inductor. It's causing the armature to move too far one direction and slam into the casing.
However, you say it only happens when you backup your files? It could be that your backup software has a serious bug in its program with is somehow causing the HD armature to try to access areas of the platters which don't exist.
Last edited by genius_at_work; 18/02/07 06:08 PM.