A disk unit (1) comprises a circular information disk (2) accommodated in an enclosure, at least one major wall (6) of the enclosure being formed with a slot (11) for an information write and/or read head of a disk drive apparatus, which slot is closed by means of a sliding plate (12) of a slot sealing device when the disk unit is disposed outside the drive unit, and which slot can be opened by sliding the sliding plate from the closed position to the open position when the disk unit is inserted into a drive unit. An actuating member for moving the sliding plate is constituted by a lever (13) which is journalled on the major wall (6) in which the slot (11) is formed, in such a way that a transmission ratio greater than unity is obtained between a free end (15) of the lever which is externally accessible to the drive unit and a second free end (16) which is connected to sliding plate (12).