USB mass storage is a class of USB devices which allows access to their internal data storage by using a predefined protocol specified by USB-IF forum.
Typical examples for Mass storage class devices are:
- External hard drives (HDD)
- External optical drives (such as CD or DVD drives)
- Portable Flash memory devices (USB sticks)
- Solid-state drives (SSD)
- Digital cameras
- Card readers
- Mobile phones
USB Mass Storage Class Bulk-Only (BBB) Transport protocol is mostly used for mass storage device communication.
Reference - USB-IF