There is a utility called os-prober that scans for other OSes than the
one os-prober is running in. This utility is called by update-grub. but
I think this has been modified from Debian 12 Bookworm on.
please verify if os-prober is installed:
$ dpkg -l os-prober
if it is not installed, install it:
$ sudo apt install os-prober
verify it is active:
$ grep GRUB_DISABLE_OS_PROBER /etc/default/grub
if the result of the previous command is not void, then edit (as root or
privileged user) the /etc/default/grub file to comment out the line
containing GRUB_DISABLE_OS_PROBER
plug your USB disk containing your FreeBSD install
then run update-grub (Debian script to launch grub-mkconfig) , it will
call automatically os-prober
This should be sufficient. If your FreeBSD install does not appear in
Grub menu, perhaps you will have to tinker a bit ("multiboot manual
config" section of the grub doc here:)
https://www.gnu.org/software/grub/manual/grub/grub.html
