Let's suppose that you've installed GNU/Linux with its GRUB. Now you had a previous installation of FreeBSD, but you can't enter to it. The solution it's simple. Edit
and add to it this part:
Now exit and see if it worked.
Code:
grub.cfg
Code:
menuentry "FreeBSD" --class freebsd --class bsd --class os {
insmod ufs2
insmod bsd
set root=(hd0,1)
kfreebsd /boot/kernel/kernel
kfreebsd_loadenv /boot/device.hints
set kFreeBSD.vfs.root.mountfrom=ufs:/dev/ada0s1a
set kFreeBSD.vfs.root.mountfrom.options=rw
set kFreeBSD.hw.psm.synaptics_support=1
}