What filesystem is on the USB drive? With that small amount of data, if it's EXT2/3, NTFS or FAT32, I'd just hookup the USB drive to the front backup port and then configure a job under Backup in Frontview to backup Volume C to the USB backup disk. You should specify a path on the USB disk, e.g. a folder such as "nasbackup" so that the backup job doesn't overwrite your data already on the USB disk.
You may wish to enable fast USB writes first: http://www.readynas.com/forum/faq.php#How_do_I_improve_write_speeds_of_USB_External_Drive_attached_to_ReadyNAS%3F
