Shock, Drop
Any external shock occurred while the drive is in operation (Power on) may cause physical damages to critical components such as Head, Disk, Motor, and etc. In non-operation condition, the impact is often limited to Head damage. If you acknowledge any abnormal operation after a shock occurred, never turn power on. Data recovery works for shock damage case should go through complicate and sophisticated processes which may include multiple component replacements under highly clean environment, so you should consult a specialist.