When you are not able to format you pen drive/flash drive then just follow this simple steps
1) Connect your pen drive/ flash drive with the usb port.
2) Now right click on “My Computer” and open “Manage”.
3) Now you will see “Disk Manager” or “Disk Management” click on that.
4) You will see all your drives, now right click the drive in which you have connected your pen drive.
5) Select “Remove Letter”.
6) Now just remove the pen drive/flash drive from the usb port and again connect it.
7) Now right click on your pen drive/flash drive and select format, now you will be able to format the pen drive/flash drive.