When you specify the option --device-map (see 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 is a drive specified in the GRUB syntax (see Device syntax), and file is an OS 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 gets the order wrong.
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 `#'.