Error 21: Selected disk does not Exist. More detailed information would be needed before you could get a definitive answer. I had this error on my computer and it was just a loose connection from the power supply to the first hard drive. What were you doing immediately prior to getting this error, had you installed any new software, hardware, etc.?
You may not need to re-install Grub. If you do
su (hit enter)
you will get the grub prompt
grub> root (hd0,1)##this command shows boot partition for your system as first drive,2n partition
setup (hd0)##this writes Grub to mbr of first drive
That is not likely your setup so you need to make changes. If you are not sure how, post again with results of "fdisk -l" command. Also post any changes you may have made recently, if any. This would probably be the way to go rather than re-installing grub.
Last edited by yancek; 04-30-2008 at 08:34 PM.