I have a somewhat similar problem. I have a Intel Based 24- inch, early 2008 iMac [3.06 GHz Core 2 Duo Processor, 4 GB Memory, NVIDIA GeForce 8800 GS 512 MB] where the the original 1TB hard drive failed. I replaced it with a WD 2TB drive. I cloned another iMac that was running 10.8.4 onto a firewire 1TB Maxtor Drive and then cloned this drive to the new WD 2TB drive. The WD 2TB drive will not boot; however, the Maxtor 1TB Drive will boot as an external firewire device. Any suggestions?