Abstract: A flexible disk cassette is provided and includes improved structure for stabilizing flutter of the disk upon rotation of the latter within the cassette, particularly at the access slot where the disk may be coupled to read/write heads for data recording or reproduction. The cassette includes a flexible data storage disk coated on both sides with a data storage recording medium and a pair of cleaning liners each positioned on a respective side of and in selective contact with the disk. The disk, sandwiched between the liners, is positioned in the cassette between a pair of spaced-apart covers bonded to one another. Respective radially extending access slots are provided in the covers to provide access of the read/write heads to the data storage medium on the disk. Positioned adjacent each radial edge of the access slot and facing the respective liner is a flat-surfaced, raised portion or land.