Does linux grub have the ability ( automagically ) ( without maunal intervention) be able to identify a BSD partition in another HDD ?
I don’t know whether the os-prober script can handle that, but GRUB itself is capable of booting BSD, Solaris and a whole other variety of operating systems.
Either way, here’s the manual. 
Thank you for the fast reply…
But my question is exactly , why does the os-prober script not be able to identify such installation ?
I don’t really know, but it’s possible that the BSD OS was not installed in UEFI mode. One must never install UEFI and non-UEFI operating systems on the same drive.
Putting them on different drives does normally work.
My machine is almost 10 years old so the UEFI detail does not apply here…
Nonetheless as I wrote in another topic the latest GRUB os-probe script is giving me some headaches by not correctly identify some other Linux systems …