The trick depended on a hole in the jacket
A 5¼-inch floppy disk was a flexible magnetic sheet inside a square plastic sleeve. Many disks sold as single-sided could still store data on the reverse face; the label described the side the manufacturer had tested and guaranteed, not necessarily the only usable surface.
To reach that other face, a user turned the disk over and inserted it again. But the drive’s write-protect sensor expected a small notch in the sleeve. Without the notch, it treated the disk as protected and refused writes. Users cut a matching notch on the opposite edge, using a purpose-made punch or, less neatly, scissors.
One disk, two recording surfaces
The original notch had a practical job: it let the drive detect whether writing was allowed. Adding a second one fooled that sensor when the disk was flipped. The drive could then record on the formerly unused face, so the disk’s usable capacity could approach twice its single-sided capacity.
This worked best with drives designed to read disks inserted either way and with media whose reverse side was reliable. A manufacturer had not necessarily tested or certified that side, so a “flippy” disk could be less dependable than a true double-sided disk. The trick also required handling the disk carefully: dust or damage to the exposed magnetic surface could spoil data.
The clever economy was not that a notch created storage. It was that the disk already had a second surface, while the sleeve and drive’s protection sensor made using it inconvenient. A small cut turned that overlooked face into extra room for files.
