3,5'' floppy disk
The 3,5'' disks had, by way of their rigid case's slide-in-place metal cover, the significant advantage of being much better protected against unintended physical contact with the disk surface when the disk was handled outside the disk drive. When the disk was inserted, a part inside the drive moved the metal cover aside, giving the drive's read/write heads the necessary access to the magnetic recording surfaces. Adding the slide mechanism resulted in a slight departure from the previous square outline. The rectangular shape had the additional merit that it made it impossible to insert the disk sideways by mistake, as had indeed been possible with earlier formats.