See:
 http://wiki.debian.org/DebianInstaller/MultipathSupport
first.

In order to install the patched grub as easily as possible from within the
d-i's shell you can do:

	wget http://tinyurl.com/mpgrub/inst_i386.sh
	sh inst_i386.sh

This will fetch and install the latest grub from this directory.
  -- Guido Günther <agx@sigxcpu.org>