Un-mount partitions
Use lsblk command to view all partitions on your USB key drive.
lsblk # sdb 7:18 1 14.9G 0 disk # ├─sdb1 7:19 1 7.7G 0 part /media/username/usb volume name # └─sdb2 7:20 1 4.9G 0 part
Un-mount all partitions on your USB key drive.
umount /dev/sdb1 umount /dev/sdb2 umount /dev/sdbX
Copy ISO
The command below is destructive and will wipe your entire USB drive with the content of the ISO file(e.g. input.iso). Make that you are pointing to the correct device path of your USB key drive(e.g. /dev/sdX.
dd bs=4M if=/input.iso of=/dev/sdX conv=fdatasync status=progress