This is to provide the Linux instructions to write ISO9660 CDROM/DVD image onto a thumbdrive. This is a general process applied toward the Ubuntu 18.04 ARM64 ISO. But, the command works for others too.
Environment: Linux based system with enough drive space or network accessed drive to hold an ISO image. You must have root access. You will need a qualified thumbdrive with more than 8GB space
Log into the Linux system
Become root (I use 'sudo bash').
Locate the desired installer ISO. We will call this <ISO-PATH> for reference
Insert a USB thumbdrive into a USB port.
Locate the thumbdrive device node (devnod):
$ cat /proc/partitions (it often is the last one displayed)
$ cat /proc/partitions
major minor #blocks name
259 0 1000204632 nvme0n1
259 1 524288 nvme0n1p1
259 2 983037952 nvme0n1p2
259 3 16641024 nvme0n1p3
8 0 15633408 sda (we will use this devnod in the example)
8 1 15632384 sda1
Ensure the drive is not mounted
$ umount /dev/sda?
Now that we are prepared. We can do the one needed command:
$ dd if=<ISO-PATH> of=/dev/sda bs=2048M conv=fsync
Go have a cup of coffee and doughnut.
Then, you are done. Remove the thumbdrive and package for shipping.