Look at both drives in Disk Utility - if the SD copied drive's partition scheme is MBR (Windoze Master Boot Record) you will not be able to boot from it, and it won't appear as a selectable drive in the Startup Disk preference panel nor with an Option-boot.
- PPC Macs need to be APM (Apple Partition Map).
- Intel Macs like GUID best (Group User ID) but will boot from APM as well.
You can select the partition format in Disk Utility under the Options button below the partition map in the Partition panel.
-- Marc