Comment 6 for bug 432254

os-prober doesn't require that GRUB 2 is installed - it's a standalone
program, in fact not originally written for use with GRUB 2 at all,
which just needs to be run as root. If it's producing no output, it's
probably because it didn't detect anything (there'll be verbose output
in syslog).

I don't think os-prober has any code to detect BSD systems at all right
now. http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=234470 was filed
some time ago about this and I don't see any BSD-related code in the
source.

It would probably not be very hard to add this support, but I don't have
any BSD-based systems to hand. Could you provide technical details of
how to detect FreeBSD reliably? For example:

 * What partition types might it use?
 * What file names might we look for?
 * How could we distinguish FreeBSD from other BSD variants?
 * Any other gotchas you know about?