Apparently not every SSD/USB3.0 converter is supported by the Armbian_5.73.190128_Rockpi-4b_Debian_stretch_dev_4.20.0 version. I have now 3 days a Renkforce SATA docking station CLONER with 2 drive bays and USB 3.0 port in use: both slots contain a WD Red with 3GB each, both are recognized and run reliably.
Under Armbian_5.67_Rockpi4b_Debian_stretch_default_4.4.154_desktop_20181210-gpt the plugged SSD was still wonderfully recognized. Obviously not all drivers were taken over in the new version?!?
I conclude the topic as solved.