My old server was Centos 4.8 and it is on a HD that is NOT connected. The new Centos 5.5 is the active HD. Everything appears to be working, so my intent is to jumper the old HD as a slave, wipe it clean and have it as a backup drive. Since my Linux skills are minimum, I thought it best to heck with the experts to find the best (safest) way to accomplish my goal.
It should be okay to do as you describe. If the disk is LVM based, see my comments below. If it's not LVM based, the worst that could happen is the system will see the partitions and try to mount them under /mnt or some such. There's no worry that the system will mount the old partitions over top your existing ones.
A quick fdisk to remove any old partitions, create new, and your off and running.
I assume that the old drive will auto-mount, however I am not sure if it will be identified as a separate due to the Volume system (I am not near my server at the moment and I cannot remember the exact name that is used for the HD configuration). I would notwant to format the drive and find out everything was formatted.
If the Volume system you refer to is LVM, then you shouldn't have any issues with the drive auto-mounting. Been a while since I did this but I believe you have to tell LVM about the volume group on that drive before the logical volumes can be mounted.