A standard 5¼-inch floppy disk of the 1980s was delivered from the factory with a notch on the right-hand side. Storage space per single-sided floppy disk on the C64: 166 kByte. A light barrier or mechanical switch in the floppy drive scanned this notch. If the light barrier was interrupted because there was no notch (original games and software), the medium was considered write-protected. An opaque adhesive strip could do the same.
With a diskette punch, it was possible to punch a second rectangular notch in a 5¼-inch diskette. With the Commodore 64 or the Apple II, whose standard floppy disk drives were equipped with only one read-write head for the bottom side, it was thus possible to use the second side additionally and double the storage capacity.
Picture: Andreas Frank
If the second side of a two-sided diskette was to be formatted in a diskette drive like the VC1541, it first had to be punched on the left side and then inserted into the drive with the label facing downwards. Otherwise, the new disk notch could not be read by the drive.