retrocomputing

For 3.5” floppy, an infrared LED and light sensor is used. If write protect slider is in the closed position, the light is blocked, and you can write to the disk. If the slider is in the open position, light passes through, and the disk is read-only. For floppies that were manufactured specifically to distribute software, they’d sometimes not have a slider at all, so you could never accidentally overwrite the disk. (At least, not without taping over the write protect hole.)

Later 3.5” floppies would have two holes, on either side of the label. One was the write protect hole, and the other identified the disk to the drive as a 1.4 MB high density disk, as opposed to the earlier 800KB disks.