Rockpi4c+ with Debian 11, how to boot from M2.SSD

I managed to get Debian11 run on rockpi4c+ v1.2 board from sdcard. Now I want to boot from M2.ssd.
The Debian OS has no cmdline.txt on /boot where I could instruct the system through partuuid=xxxx
that rootfs is on M2.ssd.
When I make a disk copy from sdcard to M2.ssd and remove the sdcard, the board does not boot at
all. So the sdcard is required to initiate the process, but how can I direct the system that the /root
partition will be on M2.ssd ? The radxa wiki for Debian is not up-to-date in this regard…

Any hint is welcome!

Helmut