`' means the parameter is optional. device should be
either `fd' or `hd' followed by a digit, like `fd0'.
But you can also set device to a hexadecimal or a decimal number
which is a BIOS drive number, so the following are equivalent:
part-num represents the partition number of device, starting
from zero for primary partitions and from four for extended partitions,
and bsd-subpart-letter represents the BSD disklabel subpartition,
such as `a' or `e'.
A shortcut for specifying BSD subpartitions is
(device,bsd-subpart-letter), in this case, GRUB
searches for the first PC partition containing a BSD disklabel, then
finds the subpartition bsd-subpart-letter. Here is an example:
The syntax `(hd0)' represents using the entire disk (or the
MBR when installing GRUB), while the syntax `(hd0,0)'
represents using the first partition of the disk (or the boot sector
of the partition when installing GRUB).
If you enabled the network support, the special drive, `(nd)', is
also available. Before using the network drive, you must initialize the
network. See Network, for more information.