5 1/4" floppy disk drives had little sensors that would detect a notch in the side of the rectangular disk sleeve (well outside of the round magnetic disk inside). Open notch meant "writeable". Manufacturers would sell "one sided" disks cheaper with missing notch for the backside of the disk to prevent using it. You could use a hole punch to pierce the soft plastic sleeve and make a "writable" hole at the correct spot. The disks inside were identical on both sides, there never was a "one-sided floppy disk", technically. This was during the "C64 and everybody got the games by exchanging floppy disks on the school yard" phase of home computing (ca 1985). Prices for floppy disks mattered a lot back then.