An adaptation device includes a lower casing member including a connection interface and forming a bottom slot in a front bottom, at least one retention flange on side walls, and pivot receiving holes in rear portions of the side walls. A retention board forms a connection interface receiving opening in a front end, rotation shafts on a rear end, and pawls on a bottom. The shafts are movably received in shaft slots defined in a rotary board. Pivots are formed below the shaft slots for being received in pivot receiving holes defined in the lower casing member. The rotary board has a hold-down plate. With a hard disk drive placed on the retention board and connected to a connection interface, the cover and the rotary board are closed forward to have the hold-down plate holding down the hard disk drive. The cover is then moved backward for fixation.