In the olden days, when floppy disks were floppy and anything that ended with "megs" was but a dream, we had to deal with write-protect notches.

On 5.25-inch (and 8-inch, if you're old enough) floppy disks, in order to prevent writing data to a disk, you had to put a small sticker over a little cutout on the side edge of the square housing of the removable media. That little cutout went to a mechanical switch that determined if a floppy was, in fact, writable.

   |            |                                 
   |     __    (  <--- Write-protect notch        
   |    (__)    |                                 
   |            |                                   
   |     __     |
On old Double Density, Single Side floppies, frugal folks discovered you could make the flip side of a floppy usable by taking a hole punch and making a similar notch on the other side. It worked most of the time, but disk companies did not verify there were no media errors, so you were taking chances with your data by doing so. It did work most of the time. The older floppies were rather hardy. I have some TRS-80 floppies from 1981 that still have no read errors. I'm talking 380K floppies, and yes, that's K for Kilobytes.
alex says: re write-protect notch: Write-protect notches are back in vogue with SD cards.

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