Inexpensive and widely used, most computers still have a floppy disk drive.
There also used to be an 8” floppy disk in 1971 (100k to 500k). 5.25” in 1976 (100k to 1.2MB). 3.5” in 1980 developed by Sony (400k, 800k, 1.4MB, special custom formats can give 2MB). http://oldcomputers.net/floppydisks.html