when do you get that error?
you might not need to reinstall you system at all (if you didn't already do so). if partition magic did not mess up your old rh9 installation, you might be able to fix your system using rh9's rescue system (if there is any - i've never used rh... you need to boot off of a installation cd and see if you can choose 'rescue system' or something similar). the problem is that your root partition used to be on /dev/hda2 (or something similar, depending on your hardware). by merging the first and second partition /dev/hda2 is now part of /dev/hda1 and, therefore, the complete mounting process is messed up and doesn't work any more. what you need to do is change ALL entries in /etc/fstab and your boot-loaders config file according to your new partition scheme. if it still does not boot try the root=/dev/hda1 kernel option at boot-up (you need to append this option at your boot-prompt).
if you don't have a rescue system on your rh-cds, try using a knoppix live-cd and mount /dev/hda1 in read-write mode (see knoppix documentation on how to do that) and make the changes there.