After installing Centos on a server with another OS installed on it (windows in my case), i received this error message on boot:
Error 21 Selected Disk does not exist
First of all, let’s reed what gnu manual says: ftp://ftp.gnu.org/old-gnu/Manuals/grub-0.90/html_node/grub_105.html
21 : Selected disk does not existThis error is returned if the device part of a device- or full file name refers to a disk or BIOS device that is not present or not recognized by the BIOS in the system.
Besically grub is not finding a disk to start boot.
How to resolve grub Error 21 Selected Disk does not exist
It is not so difficult as it seems to be.
Stop boot before server start to load OS (in my case Windos XP) and enter into GRUB Menu
Now press “e” to edit grub menù and change root location from (hd0,1) to (hd0,0) save and carrie on with boot.
Now it is important to edit grub.conf file with the same value
title CentOS (2.6.32-431.5.1.el6.x86_64)