Abstract: A flexible magnetic recording medium contained in a protective jacket is used for storing digital computer information. Such a device requires a write/erase inhibit function to preserve the previously stored information. This invention provides that function by the use of a continuously lockable shutter which can be inserted into the containment jacket. The jacket structure provides the shutter-locking channel by using an outside jacket edge and the side flap sealing embodiment to apply lateral pressure, thereby confining the enclosed shutter to slide along the jacket margin. Once so installed into the jacket, movement of this shutter is controlled through an access slot provided in the containment jacket, which slot is remote from the normal notch provided in the jacket, and is unemcumbered by any physical projection in the notch or jacket.