The capacity of a Sony Micro Floppy Disk was 437.5 Kbytes.

With recent stories of the U.S. military spending billions of dollars on keeping computer systems from the 70s running, let’s take a look back at the history of the 3.5 inch MFD (Micro Floppy Disk). In 1979, Sony began research on a storage device for a new word processing system. The goal was to develop a smaller floppy disk with a large storage capacity. They chose to put the disk in a plastic shell to increase its durability.

The Series 35 Word Processor resembled an Apple Macintosh

In December of 1980 the Sony Series 30 Word Processor was announced, this was the first product to use a 3.5 inch disk.

The SMC-70 could display kanji with a custom version of BASIC.

Announced in 1982 the SMC-70, was graced with two 3.5 inch drives. With advanced graphics capabilities, this computer was designed for broadcast use. Sony’s Micro Floppy Disk would ultimately become the basis for a new international storage standard. The legacy of the original Sony 3.5″ disk lives on today in many computers, from the iconic Apple Macintosh, to the Atari ST and Commodore Amiga.

Source: US nuclear force still uses floppy disks – BBC News