Sony 3.5” disk, from the April 1981 issue of Byte (at archive.org). The editorial was breathless:
Another Sony breakthrough is a new miniature floppy-disk system (see photo). Each disk measures 8.9cm (3½ inches) in diameter and holds over 800,000 bytes! The disk resides in a rigid housing for protection.
One of the first major users of the Sony disk was the Macintosh, but the decision was not without its drama.