Abstract: A portable computer includes a central processing unit, a monitor unit laying on and pivoted to the central processing unit, and a hard disk drive unit fitted in the storage chamber of the central processing unit. The storage chamber has a bottom wall and a side wall assembly which has two locking holes. The hard disk drive unit includes a housing, two insertion elements mounted movably in the housing, a rotary actuating unit mounted pivotally on the central processing unit, and a spring unit which biases the insertion elements to extend from the housing to engage within the locking holes of the storage chamber so as to retain the hard disk drive unit in the storage chamber. When the rotary actuating unit is turned upward, the insertion elements retract into the housing, thereby enabling the hard disk drive unit to be removed from the central processing unit.