Abstract: An insert for blocking the disk write arm notch of diskettes used in small computers, in word processors, and in other office machines, comprises an elongate slender insert positioned for sliding movement in the edge portion of the envelope of a diskette to block the write-protection notch thereof so that the read-write-erase functions of the machines are prevented from operation, so that accidental erasure or writing over of previously stored information will not occur. The insert is pushed in to block the notch, and withdrawn to unblock the notch, and may be shaped, colored, or otherwise distinctive in appearance at its outer end to identify the diskette.