It sounds like you are putting the new drive where the old drive was. If that is the case, when you first start to boot and grub looks for /boot, it isn't there, at least not where it was anyway.
You should be able to put in the new drive and then boot the Redhat CD and enter the rescue mode. I am not familiar with the steps, don't use Redhat, but point grub to the new drive for /boot and the grub files. From what I have read it is pretty easy to redo the bootloader in Redhat.
It should work fine after that. If you are doing what I think you are doing.
edit: left out the part about booting the "CD". Sort of have to clarify a bit. Read bottom of sig for reasons.