Issues Booting to New Leopard Install

I recently had warnings about my backup external disk, so I bought a brand new disk to use in my old Mac Pro, with the intention of moving the old disk over as the backup. In the process, I decided to go back to Leopard, since this machine functions mostly as a back-up disk for my MBP and iMac (both are much newer).

So, the installation went fine, but when I removed the original boot disk (which has Mountain Lion on it), my old mac never could find a bootable partition. I'm assuming it was able to boot before because the install DVD was in the drive, which I removed the other day.

So, the question is: Is there any way I can make an existing disk bootable (i.e. the new one) without losing any info on the disk or do I need to do something to the disk so that it will boot when the other disk isn't present?

When I go to "Start up disk" in ML, I only see the ML disk, not the Leopard disk.

As for formatting the HDD properly, I let Apple do that

That's what I'm assuming is wrong. Does it need to be formatted in a particular way? When I go into Disk Utility, I'm not seeing too many different things. But, I'm not sitting on a virgin disk. It has been partitioned and is in use ATM.

Also, when I look at "Info" for the partition (which is the entire disk), it does say "Bootable: Yes"

The format says: GUID Partition Table

I just wonder if the original install wrote to the boot sector (or EFI whatever it is on macs) on the original boot disk. I think it was in either bay 1 or at least a lower bay than the new disk is in.

I just want to avoid reinstalling if there is some simple command to copy boot loaders over, and I'm just not seeing anything with my google searches...probably because I don't know the correct terminology.

----------

Apple didn't do the actual install. I did. I just meant that Apple wrote the installer I used to install Leopard onto a machine that already had a functional version of Mountain Lion on it (but not on the same physical disk). So, the disk was formatted by Apple, indirectly

So, I know what the problem was. The disk is faulty, apparently. I've reinstalled 3 times, and after a few days, it just loses the ability to boot into that disk.

Is there any low level formatting I can try before I try to get it replaced? It's a new disk (maybe 3 weeks old tomorrow...) It isn't sending out SMART errors, which is why I was fooled into thinking I had done something wrong with the previous installation.