From info grubThe map between BIOS drives and OS devices
==========================================
When you specify the option `--device-map' (*note Basic usage:

,
the grub shell creates the "device map file" automatically unless it
already exists. The file name `/boot/grub/device.map' is preferred.
If the device map file exists, the grub shell reads it to map BIOS
drives to OS devices. This file consists of lines like this:
DEVICE FILE
DEVICE is a drive, which syntax is the same as the one in GRUB
(*note Device syntax:

, and FILE is an OS's file, which is normally a
device file.
The reason why the grub shell gives you the device map file is that
it cannot guess the map between BIOS drives and OS devices correctly in
some environments. For example, if you exchange the boot sequence
between IDE and SCSI in your BIOS, it mistakes the order.
Thus, edit the file if the grub shell makes a mistake. You can put
any comments in the file if needed, as the grub shell assumes that a
line is just a comment if the first character is `#'.
Installing GRUB using grub-install
==================================
*Caution:* This procedure is definitely deprecated, because there
are several posibilities that your computer can be unbootable. For
example, most operating systems don't tell GRUB how to map BIOS drives
to OS devices correctly, GRUB merely "guesses" the mapping. This will
succeed in most cases, but not always. So GRUB provides you with a
user-defined map file called "device map", which you must fix, if it is
wrong. *Note Device map::, for more details.