Abstract: An assembly comprised of a flexible, magnetic recording disk and a jacket or envelope which houses the disk in a manner such that the disk can rotate in the jacket under the influence of a conventional disk drive. The jacket has a notch at one corner thereof to provide a write enable function for the assembly when used with the disk drive. A rigid member is provided adjacent to the notch to close the opening formed thereby between the two sides of the jacket and to reinforce the corner to prevent structural damage to the jacket when used with the disk drive. The jacket is formed from an initially flat sheet of material and, except for the aforesaid rigid member, there is no structure between the outer periphery of the disk and the adjacent margin of the jacket.