How can I tell the difference between unlabeled DD and HD diskettes?
3.5″ HD (1.4MB) diskettes come with a square notch in the upper left corner. DD (800kB) 3.5″ diskettes do not come with this notch. In the early days of PC computing, some PC users punched or drilled notch holes in DD diskettes and used them as HD diskettes. If a 3.5″ diskette has a circular notch in the upper left corner, it is likely to be a DD diskette. DD 5.25″ (360kB) diskettes look very much like HD 5.25″ (1.2MB) diskettes; however, HD diskettes seem to almost never have a hub ring, while DD diskettes usually do. The hub ring may be white paper, etc. and easy to spot or cut from the same material as the diskette and barely noticeable. This difference has been mentioned by Peter Norton (of Norton Utilities fame) in one of his books. The hub ring makes it easier for the Apple Disk Drive II and other older DD 5.25″ drives to clamp and hold the diskette. Older DD drives also tend to damage the center when there is no hub. If the hub ring of a DD diskette has fallen off due to age, it
