Sorry to resurrect an old thread!
As a new Armbian user on Rock64, I found this thread while searching for a guide on how to boot from a usb device. After playing around with partitions and the armbian-config utility, I realized that after copying rootfs to the usb device, the board will still read from the /boot partition on the sd card, then will load rootfs from the external drive. If the board is powered on without the sd card, boot will fail. This method works fine for my purposes. If there is a way to configure the rock64 to read from a /boot partition on a usb device, I haven't found one.
I hope this helps any other new folks like myself who want to run Armbian from usb/ssd on Rock64. Cheers!