An external casing includes a housing housed with a data storage device and formed with an opening. An interface module is disposed in the housing, and is connected electrically to the data storage device disposed in the housing. An external electrical connector is connected electrically to the interface module. A foldable cover member includes a fixing portion secured to a first housing part of the housing, an engaging portion opposite to the fixing portion, and an intermediate covering portion interconnecting the fixing portion and the engaging portion and covering over the external electrical connector. A lock unit is provided on a second housing part of the housing and the engaging portion of the cover member for selectively anchoring the engaging portion of the cover member to the second housing part of the housing.