A recent install of Ubuntu Gutsy 7.10 on a slave IDE disk (cable select) with an existing master IDE disk with XP Pro on it rebooted and got a GRUB Error 21. I was about to boot off the network again to go into rescue mode and look at the grub configs, but when I saw the boot menu I wondered what the boot order since I just added the disk. When I got into BIOS I saw that Primary Slave was OFF. Ubuntu had seen the disk even though the BIOS had it disabled, and since GRUB talks to the BIOS it couldn’t find the disk. Enabling the disk by setting it to auto in the Dell BIOS fixed GRUB.