A suction box for a system for transporting flat media (4) includes at least one endless belt (6), formed with a plurality of through-holes, used in a media printing machine (1) equipped with at least one printing unit (2). The suction box includes a suction device (23) to generate a vacuum, a suction compartment (16), divided into at least two distinct chambers (26, 27, 28), including a first suction chamber (28) of variable volume, in communication with the suction device (23) so as to apply the vacuum through the holes in the belt (6) to the media being transported by the belt (6), and a second chamber (26, 27) at ambient pressure. The first and second chambers (28, 26, 27) are separated from one another by a mobile partition (31, 33). The suction compartment (16) has an open upper face equipped with a holding arrangement (14, 43, 44, 46, 47, 48) allowing movement of the belt (6).