...and finally got it working
I found the soloution in Bios, where i enabled "usb 2.0 rate matching hub" and bingo
It's really over my head - but hours of searching paid off.
Thanks Ralph.
USB 2.0 Rate Matching Hub • Enables lower power requirements and manages the transition of the communication data rate from the high speed of the host controller to the lower speed of USB full-speed/low-speed devices. access with up to 2 SATA ports.