A disk cartridge comprises a disk (1) having a central hub (4) for engagement with a disk drive. The disk is accommodated in a housing (2). The housing and disk are dimensioned such that there is limited clearance in a direction parallel to and perpendicular to the plane of the disk. The cartridge comprises a clamping member (11), such as a magnet, to clamp the disk to one side of the cartridge housing when the cartridge is free from the disk drive. The housing and the disk have co-operating centering members (13, 14), preferably near the center of the disk, so as to center the disk with respect to the cartridge housing when the disk is clamped to the housing by the clamping member. In this way, the disk is always in the correct position for a next engagement by the disk drive.