green LED flashes 4 times - rpi B 3+ but works USB-boot

Hi!
I have for a long time run a rpi 3B+ by having
/boot
/
on the SD card
and all application specific stuff on a small
(80 GB) USB-disk 2.5" spinning metal
So the other day I bought a 120 GB SSD disk at amazon

And I use gparted on another Linux box to
- clean out the new SSD
- copy /boot from SD card and paste on SSD
- copy / from sdcard and paste on SSD
- copy /bnlbot from old USB-drive and paste on SSD
The whole procedure was very smooth - I did not know of copy/paste
within gparted before.
in /boot/cmdlie,txt I changed the root UUID
in /etc/fstab I did the same, changing UUID and PARTITIONUUID
to new values.
Nice - unmount from PC and put SSD in rpi via USB - with no SD card
Power on
LED flashes 4 times - seem to mean

* 4 flashes: start.elf not launched
But - the PI works,
I can SSH to it
the database works, my applications has started.
dmesg confirms boot from USB (well there is no SD-card so)
Everything but the LED seems fine
Why does it flash?
I cannot google an answer to make it not flash