Originally grub was installed to /dev/sda but now it has set to boot via /dev/sda6 FreeBSD boot, which chainload grub. Drive /dev/sda was originally drive /dev/sdb, if someone is wondering why its freebsd entry shows hd1,6

Anyway .. if wanting to load FreeBSD always the right way and disk, the best way is to use UUID for determining the right partition.

Command sudo blkid in linux tells the UUID

In the drive 2 /dev/sdb , FreeBSD is installed to partition /dev/sdb1 , which UUID is xxxxxxx

For getting FreeBSD to boot from /dev/sdb1, the line in linux´s /etc/grub.d/40_custom file is

menuentry "FreeBSD search works" {
insmod ufs2
insmod part_msdos
insmod search_fs_uuid
search --no-floppy --fs-uuid --set=root xxxxxxxxxx
kfreebsd /boot/loader
}

.. or

menuentry "FreeBSD search works" {
insmod ufs2
insmod search_fs_uuid
search --no-floppy --fs-uuid --set=root xxxxxxxxxx
kfreebsd /boot/loader
}

If using UEFI boot, the entry would be

menuentry 'FreeBSD UEFI UUID chainloader' {
insmod ufs2
insmod part_gpt
insmod search_fs_uuid
search --no-floppy --fs-uuid --set=root xxxxxxxxxx
chainloader /boot/loader.efi
}