MassStorage

USB includes the notion of Class drivers, a set of software components generally delivered by Operating System vendors that are able to communicate with not just one, but many different USB devices of a particular kind.

This enables any number of compliant devices to be plugged in and start working immediately without the need to install additional software. Several Class Drivers are already included in Mac OS for its computers with USB, including a hub class driver and a composite class driver.
Over time, Apple expects to offer new and updated Class drivers for USB. The Mass Storage Support software represents one of these class drivers and is intended to support devices that meet the USB Mass Storage Class specification. This software consists of the two system extensions USB Mass Storage Extension and USB Mass Storage Support