@gordini21

Thank you very much for the hint!
Yes, I could boot with the help of the USB-Stick.

But I do not know what to do then...
Because: In Prefs/Screens the correct graphics card is set (X1650Pro).
Do I have to copy this prefs? Is this X1650 configuration only saved for/on the USB-Stick?

The only strange issue: In prefs/monitors a wrong monitor (31kHZ) is set. But also if I enter there the correct values of my monitor, save and then try to boot with the SSD disk, I get the 'wrong screen resolution' message....