I'm having a grub problem that I can't figure out.

I have an FC5 system which started having problems with the hard drive (IDE 0 Master/hda).
So I added a second drive (IDE 0 Slave/hdb), installed FC5 on the new drive (hdb) and
copied everything off the old drive onto the new one.

I quickly discovered that things aren't behaving quite right. FC5 performs all updates
to the kernel, grub.conf, etc. on /dev/hdb1 and the grub.conf says to boot from
hd0. When the system actually boots, its using the files from the old installation
on /dev/hda1. If I copy grub.conf from /hdb1 to /hda1 and change all references
from hd0 to hd1 it all works "normally."

hda1 is getting worse, so I want to remove it from the system entirely. When I do
so and make the new drive the master on IDE 0, the system halts with a
"GRUB hard disk error" message.

- MGG

You need to reinstall grub on this new disk. Boot the system in resuce mode and install grub. Also check the devices.map file and make sure that it contains correct entries as well.

The easiest solution would be to swap master slave jumpers and install grub on the new drive.