Use dmesg after plugging in your USB thumb drive to find the device name (/dev/sdb).

# umount /media/username/devicename
# dd bs=4M if=/path/to/filename.iso of=/dev/sdb

Note, do not output to a partition (e.g. /dev/sdb1). In the past this has created a loop device instead of copying the input file to the USB thumb drive.