Re: Error 21
Here's a link containing the GNU/GRUB manual's interpretation of it plus some information I've collected on that error message, Grub error 21.
If you have IDE and SATA drives together, you might just need to edit your /boot/grub/menu.lst file with the correct hard disk number.
This link should help you access your hard disk installed Ubuntu's vital files from the Ubuntu Live CD, Mount a Ubuntu ext3 or reiserfs filesystem .
If you have IDE hard disks, make sure they are plugged in an jumpered correctly for the type of IDE cables you use. Even the CD/DVD drive needs to have the jumpers set correctly as well.
If you need specific help, please post a copy of the bottom half of your /boot/grub/menu.lst file.
Ubuntu user since 2004 (Warty Warthog)