There's what seems to me an even easier way. Sorry for promoting my own program but:
on your booted SD card, plug your USB disk in and, say it shows up as /dev/sda:
Code: Select all
$ git clone https://github.com/billw2/rpi-clone.git
$ cd rpi-clone
$ sudo cp rpi-clone rpi-clone-setup /usr/local/sbin
$ rpi-clone -l sda
The reboot boots to root on /dev/sda2 and /boot mounted with /dev/sda1.
Later, after upgrading packages, configuring or modifying files in /boot (/dev/sda1) back the USB disk back to the mmcblk0 SD card (if it is still plugged in) so that reboots will still be to USB disk:
Later, after the firmware becomes able to boot directly to USB disk, remove the SD card and (without any editing) reboot.
Now after changes you want to backup to another USB disk or SD card in a card reader, don't use the -l option to rpi-clone anymore:
There are some other complicated setups rpi-clone can handle such as a three partition USB disk cloning from/to a two partition SD card, but that takes a few more steps and/or rpi-clone options which would be confusing to get into when all we want here is a simple way to USB boot.