Most possibly, you make use of the external hard drive very often and do not take care of its enclosure. The enclosure of an HDD is used to protect the hard disk inside. There are import data on the hard disks, therefore, to protect them, manufacturers warp them up with enclosures. Also, users take care of the hard drives. On the contrary, users ignore the enclosures. Yet, being exposed to the air, the enclosures have much higher chance to get damaged.