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  
<https://www.amazon.se/dp/B077QTGTDV?ref_=pe_20773151_507143461_E_301_dt_1

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
<https://elinux.org/R-Pi_Troubleshooting#Green_LED_blinks_in_a_specific_pattern

* 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