Just because the square-cut notch in the 3.5 inch diskette is covered does not necessarily protect the diskette from writing on it. The square cut in a floppy diskette lets a small light shine on a receptor to tell the drive it is all right to write on the diskette. Conversely, if an opaque paster covers the square slot, no light can shine through the slot to the receptor and the drive won't write on the diskette-if the receptor is opposite the light.
Some disk drives have the receptor installed next to the light.
In these drives, the light shines through the slot to a mirror which reflects the light back to the receptor. An aluminum-backed paster reflects the light to the receptor as efficiently as the mirror would. One of the items Internal Audit looks at is verification of proper approvals for expenses. Signature cards are used for this verificiation. Just a friendly reminder.