the GNU GRUB manual
The GRand Unified Bootloader, version 0.97, 8 May 2005.
Once had the same ERROR CODE 21 while extracting LIVE-CD to USB-STICK and putting GRUB-1 to MBR of HD-1 (hd0,0).
The Error Code happend when I did not insert the Pendrive just to boot into Windows.
Grub could not read the /boot/, /boot/grub/, /boot/grub/menu.lst directories and files.
It is somewhat kind of same to ERROR
15 : File not found
16 : Inconsistent filesystem structure
17 : Cannot mount selected partition
21 : Selected disk does not exist
22 : No such partition
25 : Disk read error