I am guessing RH is still there and that GRUB just doesn't have it in its configuration file.
I would boot into CentOS and use one of its partitioning tools to see what partitions are on the two disks. If you have fdisk
will list all of the partitions on both disks. Otherwise, see what partioning tools you have using
Compare this against the partitions that are currently mounted (issue the mount
command w/o any parameters). I am guessing you will see one or more partitions on your slave drive (partitions that start with /dev/hdb) mounted and none mounted from your master drive (partitions that start with /dev/hda).
If this is true, create a temporary mount point (say /mnt/temp) if you do not already have one, and see if you can mount any of the partitions from your master drive on it with commands like:
mount -t auto /dev/hda1 /mnt/temp
These commands need to be run as root
. If you can mount any of these, look at their content with
This should tell you whether your RH system is still there. If it is, you just need to modify you GRUB configuration file to give you a choice of which to boot.