> Samsung EVO 850 [...] (which should be a SATA III drive?!)

Yes, it is.

> So was there a SATA I or II version of this ssd drive,
> which should only work in slot "A" ?

The Samsung 850 EVO never came as SATA1/2. And as far as I understand, the "phenomenon" with the G5 is such way that the drive either works on both ports or just on port B, but never just on port A.

> Maybe this ssd is somehow downwards compatible to the
> SATA I controller in there and thats the reason ?

Yes, apparently. As I wrote in comment #42, there are SATA3 drives that do work on SATA1 ports.