Comment 5 for bug 518826

The way this works is that update-grub runs the os-prober command, and
then also runs the linux-boot-prober command on any partitions that look
like they contain Linux. /etc/grub.d/30_os-prober takes the output of
those commands and turns them into grub.cfg fragments. I think this is
much more likely to be a problem with linux-boot-prober than a problem
on the grub side.

I don't have Arch Linux installed myself, so I'll need help to debug
this. First, please show me the output of 'sudo os-prober'. In that
output, there should be a line something like this:

  /dev/sda7:Ubuntu 9.10 (9.10):Ubuntu:linux

... but for Arch Linux instead of Ubuntu, and probably with a different
device name at the start of the line. If there is such a line, could
you please run 'sudo linux-boot-prober /dev/sda7', but replace /dev/sda7
with the device at the start of the line, and show me that output as
well?

 status incomplete