Dual BOOT Linux 3 and 4

If you've got RHEL3 on partition /dev/hda2 and RHEL4 on /dev/hdb2, then you need to edit your /boot/grub/grub.conf file like this:


# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/hda5
#          initrd /initrd-version.img
#boot=/dev/hda
default linux
timeout 10
color green/black light-gray/blue

# section to load RHEL3
title RHEL 3
root (hd0,1)                                      //  /dev/hda = (hd0), so /dev/hda2 = (hd0,1)
kernel /vmlinuz root=/dev/hda2
boot


# section to load RHEL4
title RHEL 4
root (hd1,1)                                         //  /dev/hdb = (hd1), so /dev/hdb2 = (hd1,1)
kernel /vmlinuz root=/dev/hdb2
boot





Now all this depends on how you have your partitions set up.  If you've got your root files on /dev/hda2 and your /boot files are on /dev/hda3, then you need to change the line that starts "kernel" like this:

# section to load RHEL3
title RHEL 3
root (hd0,1)                                      //  /dev/hda = (hd0), so /dev/hda2 = (hd0,1)
kernel /vmlinuz root=/dev/hda3
boot



But basically, you manage this by altering the grub.conf file.  You can find much more detailed information at this site:

http://www.redhat.com/docs/manuals/linux/RHL-7.2-Manual/ref-guide/ch-grub.html