Map the from drive to the to drive. You need to do this when chainloading an operating system such as Windows, if it is not on the first drive. For example, if Windows is on (hd1):
map (hd0) (hd1)grub>
map (hd1) (hd0)
This swaps the mappings of the first and second hard drives, tricking Windows into thinking it’s on the first drive so it can boot.