(address . bug-guix@gnu.org)
The current (chain-loader) declaration seems to only work properly when
booting in EFI mode[1], as that way you have to specify a file path to
chainload an operating system that's not compatible with GRUB such as
Windows, but this solution will not work properly when using "+1" for
the old MBR/msdos partition table that required bootstrapping, not a
filesystem path, as explained in GRUB's documentation[2].
I've asked about this issue on the guix-help mail list and someone
pointed out where the problem is on the source file[3], it's only a
matter of submitting a patch to fix it so it parses properly when the
(chain-loader) declaration has "+1" for bootstrapping instead of a path.