So I wrote the GalliumOS ISO image to the USB stick again and boot with it. Even though I press Ctrl-U to boot from USB, nothing happens.
Some searching found something about the firmware not recognising the USB ports during the boot process. There are updated firmware images at MrChromebox. I download the script with curl as directed and run it (it requires bash, not sh). It then presented me with the option of either updating the RW_LEGACY firmware which is based on SeaBIOS, or installing a full custom firmware. The former is safe so I decided to try that first.
The firmware upgrade takes and I power cycled and wait for it to boot GalliumOS. Still no joy. Even if I type Ctrl-U it does nothing.
Some searching uncovers the fact that there are actually two sets of firmware onboard, the ChromeOS loader and the legacy BIOS loader which is more similar to PC firmware. To get the legacy loader I have to type Ctrl-l.
Bingo! It boots from the USB stick, fairly quickly I must say, and gives me a live desktop. I decide to run the installer right away to put GalliumOS on the SSD.
I am presented with a dialog giving me a choice of WiFi APs to log into so that I can install and upgrade to the latest package versions as well as install any 3rd party software. But I still can't login to my AP. Is the Chromebox in a WiFi shadow? It's in the same room but almost vertically below the AP.
Ahhh, the MAC filter! I have the MAC filter enabled on my AP so that only my devices can use the AP, an additional layer of protection to the password. I added the MAC address of the Chromebox to the table, and this time I can connect.
The install was fairly quick, considering the slow speed of the USB2 stick. I ended up with the GalliumOS desktop.
Success! I think I'll call it a day. I still have to work out how to make legacy BIOS the default on power up so that I don't have to type Ctrl-l. Maybe replace the legacy BIOS with the custom BIOS, which cannot boot ChromeOS, but I don't care about that now. I must also see if I can install another SSD inside for a /home partition, 16 GB isn't a lot after the OS has taken its share.