Just to report success (at last) with getting my Pi 4 to boot from USB.
The problem I was having with the 500Gb USB drive was either due to power problems or timing, I'm not sure which. As reported in the original thread about this the Pi booted from this drive OK when it was plugged into one of the USB-2 sockets but failed to boot (from the USB) when it was plugged into one of the USB-3 sockets.
I have now put an old[ish] 320Gb drive in a SATA to USB-3 box which has it's own power supply and that works perfectly when plugged into one of the USB-3 sockets.
Having configured the eeprom to look for USB *before* SD it boots from the USB if the USB is there. This is rather handy as I was then able to mount the SD card and copy files from there to the new installation.
Just for interest (and in response to another query/suggestion here) /etc/fstab is:-
proc /proc proc defaults 0 0 PARTUUID=e258a3fc-01 /boot vfat defaults 0 2 PARTUUID=e258a3fc-02 / ext4 defaults,noatime 0 1 # a swapfile is not a swap partition, no line here # use dphys-swapfile swap[on|off] for that
I.e. the /boot and / partitions are recognised solely by their partition IDs, no device names in there at all.