The USB Mass Storage Device Class (USB MSC) is a set of computing communications protocol that runs on the universal serial bus (USB). The electronic hardware that stores information and support a protocol to send and retrieve the information over a hardware interface is a mass storage device. A mass storage device typically stores information in files.

Some of the devices that practically implement the USB Mass storage devices are external hard drives, CD and DVD reader and writer drives, various digital audio players and portable media players, gaming systems and mobile phones. Most of the current mainstream operating systems support USB Mass storage devices and therefore USB mass storage is versatile enough for a wide range of peripheral devices. Every USB communication is between a host and device, USB host is a PC and the host controls the communication. USB mass storage device contains USB device controller hardware and program code.

A USB device controller enables a USB mass storage device to share its data with other computers. Programming and designing a USB mass storage device involves a variety of interfaces, protocols and structures. Every USB Mass storage device supports two interfaces. First a USB device interface to enable the device to communicate with other USB host and secondly an interface between the device’s micro controller and CPU and storage media.

A USB mass storage device should implement four protocols namely, Generic USB protocol (every USB device should respond to requests sent by the USB host and other events) and USB mass storage protocol (every USB mass storage device should detect and respond to request specific to USB mass storage class), SCSI commands (USB hosts access mass storage device through commands developed originally for Small Computer Systems Interface-SCSI) and media specific protocol (the storage media’s controller typically supports a command set for accessing the contents of the media).

Reading and writing data to a USB mass storage device also involves understanding media structure and file system. Media structure is the information about the logical blocks, which are nothing, but the storage area in drives. File system such as FAT 16 or FAT 32 should be implemented if USB mass storage device reads or writes files on its own rather than through USB hosts.

USB mass storage supports three bus speed, low speed – with a bandwidth of 800 bytes per second and useful for keyboard and mice, full speed – to transfer data up to 1.2 mega bytes per second and a high speed – to transfer data at over 30 megabytes per second. It is recommended that USB devices support USB 2.0.

USB mass storage has four types of transfer and they are used for enumeration for control transfer, printer and scanner data for bulk transfer, mouse and keyboard data for interrupt transfer and real time audio and video for isochroous transfers.