I am familiar with USB Mass Storage mode . How exactly is USB Debug mode different from it ?
An Android device in "Mass Storage" mode behaves just like a USB flash drive: It presents itself as an block device that can be mounted (i.e "used") by the host PC. Just like an USB stick does.
Additionally "USB Debug Mode" can be enabled, which creates a new USB endpoint for the Android Debug Bridge (
adb). This allows an user to get in-deep access to the connected Android device and furthermore it allows the debugging of Apps. This is the reason why you should never attach an Android device with Debug Mode enabled to an untrustworthy PC. As USB is able to provide more then one endpoint per USB device, this "services", Mass Storage and
adb, can be used simultaneously.