Hard disks are, for what it matters to this question, made of plates spinning and a magnetic reading head, that moves to read the disk. This head is fixed in a moving arm and it's positioned very close to the disk.
See Also
Buff - Level up Everywhere | Game to Get Items IRLThe Buff Business Model – How Does Buff Make Money?7 Bad Habits That Are Destroying Your PCHow Does Buff Really Work? (Earning Points Safely & Easily) - The Buff Browser - Gaming Blog - BuffIf you shake the HD, the head might touch it's surface. So, it'll cause damage. In the long run, it'll make it worse, as the micro- and mini- and regular- damages will acumulate.
So, never shake a running HD.
