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?