The Commodore 4040 and its sibling(s), the 2040 and the European marketed 3040, were dual unit 5¼" floppy disk

Floppy disk

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

 drives for Commodore International

Commodore International

Commodore is the commonly used name for Commodore Business Machines , the U.S.-based home computer manufacturer and electronics manufacturer headquartered in West Chester, Pennsylvania, which also housed Commodore's corporate parent company, Commodore International Limited...

 computers. They used a wide case form, and used the IEEE-488

IEEE-488

IEEE-488 is a short-range digital communications bus specification. It was created for use with automated test equipment in the late 1960s, and is still in use for that purpose. IEEE-488 was created as HP-IB , and is commonly called GPIB...

 interface common to Commodore PET/CBM

Commodore PET

The Commodore PET was a home/personal computer produced from 1977 by Commodore International...

 computers.

These drive models used a similar single density, single side floppy data storage format to that used by the Commodore 1541

Commodore 1541

The Commodore 1541 , made by Commodore International, was the best-known floppy disk drive for the Commodore 64 home computer. The 1541 was a single-sided 170 kilobyte drive for 5¼" disks...

drive, but with a slightly different data marker indicating which model originally formatted the disk. The low level disk format was similar enough to allow reading between models, but different enough that one series of drive models could not reliably write to disks formatted with one of the other model series.