Not able to read device description of pen drive via USB interface.

Some pen drives include additional devices (security, etc). Simplistic software like most embedded libraries can't deal with this. Sometimes USB hub(s) are also included. I'd suggest trying your code with a variety of different brands, models, sizes of pen drives and use the ones that work. That's easier than debugging a more complex pen drive. If your device needs to work with any generic pen drive, I'd suggest purchasing a commercial USB and file system stack for embedded devices rather than relying on code from a demo.