Map the drive from_drive to the drive to_drive. This is
necessary when you chain-load some operating systems, such as DOS, if
such an OS resides at a non-first drive. Here is an example:
grub> map (hd0) (hd1)
grub> map (hd1) (hd0)
The example exchanges the order between the first hard disk and the
second hard disk. See also DOS/Windows.